I have a little problem in the Ars Magica saga in which I'm playing.
I have take the virtues Strong Faeries blood; Sidhe, presence +1; but my GM make all the npc réact negativly when they see my character, the book say that the character look fascinating and alluring and that you can hide you faerie's nature but in this case it look more like everybody want to throw me stones in my face?
How do you use this major virtues in your saga?
I'm not sure how the virtue was intended to work, but extrapolating from what I do know about the rules, I get the impression that the physical features granted by Strong Faerie Blood are supposed to be exotic and possibly alluring for most character types.
If, however, the character has The Gift (or worse, Blatant Gift) it alters peoples' perceptions causing them to perceive the character as an inhuman monster which must be put to the torch for the good of the community.
.02 Mythic Pounds.
I agree that it sounds like you've got the Gift as well, with no counterbalance.
The way the Gift works, as I see it, is that it's like dramatic camera angles and music in a movie, where the mage is being foreshadowed as something scary. Everyone who sees the mage hears the ominous music and feels a foreboding sense of something being really not right.
So, while a Sidhe character may be fascinating and beautiful, there's an undercurrent of menace and ill-feeling from the Gift that mundanes perceive - more than a real faerie, possibly.
As a player of a Flambeau maga with both Blatant Gift and Strong Sidhe Blood, I'll post my 2 cents worth.
Look at the Virtues/Flaws seperately.
Strong Faerie Blood gives +1 Prs. This is a clear cut advabtage for you, and should work equally well for you in all (well, most) circumstances. It's described as something foreign, but alluring. So I'll accept that sometimes, someone will react negatively to it. I'm talking fanatic mobs here, someone already scared out of theri wits and/or brainwashed, to react poorly to such unnatural features.
While the Gift by itself confers -3 or -6 to social interaction, depending og whether it's Blatant or not. This is a clear cut disadvantage, and should hamper you.
Even if the +1 Prs should always help, this still leaves -2 to -5 on the negative side.
Still, this should not form mobs with torches and pitchforks, throwing rocks and trying to drive you out with the Words of the Lord.
It's actually worse than that: people will react negatively. The -3/-6 penalty comes on top of having to overcome that reaction.
Not systematically, but it could happen with some regularity, especially if the character has some added weirdness on top, like the beauty of the Devil. 
Except that in Mythic Europe, the villagers know that there are faeries at the bottom of the garden. and actively leave out milk and the last bit of corn for them. Yes, I agree that there should be a social stigma for being obviously marked by a Realm in some way but I don't think it should extend to lynch mobs. They might be more likely to form, I agree, and it would depend heavily on where you were, but anything over extra suspicion and the daughters being locked up seems extreme.
In the case of Sidhe blood, the character is explicitly more appealing than most - positive Presence cannot be a bad thing. More generally, it's a virtue and that sort of extreme reaction is more of a really huge Story Flaw.
I favour the interpretation that Strong Faerie Blood (Sidhe) makes one more alluring, fascinating, and exotic.
The Gift works on a subliminal level. It isn't filtered directly through conscious perceptions. To use the vernacular, one could say that it causes one to give off "disturbing vibes" that people and animals not accustomed to the Gift (such as those indigenous to Slavic Faerie areas, mundane as well as Fay, are) react to negatively. Strong Faerie Blood does not have this effect unless one also has the Magical Air and Offensive to Animals Flaws. Even then, however, those of Magical or Faerie Realms who lack Might scores wouldn't react adversely.
Another, related Minor Flaw which would be appropriate for those with Strong Faerie Blood would be "Disfigured". It represents those with a very strange appearance, regardless of their actual Presence, and has included those with animalistic or truly bizarre countenances at least since the original version of "Faeries" was published back in late 2nd Ed.
One should remember that all sorts of tales are told about different peoples in Mythic Europe--including those who are ostensibly mortal and human. Quite a few ascribe weird features to them, yet while they may be thought "foreign" they could still be considered attractive. On a practical, RL note, it was a common effect of both diet and custom in many areas of Europe (and around the world) for people to develop physical distortions, particularly regarding the shape of the head and face (head-binding and the use of cradleboards are two major causes). These individual may or may not qualify for the Disfigured Flaw, but they definitely wouldn't suffer the reactions of the Gift.
Culturally, people taught to respect the Sidhe would tend to treat anyone who might be perceived as a member of that Faerie "race" with extreme politeness and care. Both in hope of currying favour from them and to avoid earning their wrath.
It's a slightly different story with other types of Strong Faerie Blood, but the comments regarding their "exotic" qualities and the customs of the age remain.
As the others have commented.
I would play that the Sidhe blood should normally be a positive factor. On average, people would react more favourably to a character with Sidhe blood than without (that's why they have +1 Pre). Although, of course it might depend on the circumstances. If, for example, some people in a village have been attacked by faeries recently, then your character might be viewed rather hostilely, if he happens to wander into that village.
On the other hand, if your character also has the Gift (and doesn't have the Gentle Gift), then the negative social impact of the Gift should overwhelm the positive aspect of Sidhe blood. But remember, if your character does have the Gift, when dealing with other characters who have Parma Magica the negative social effects of the Gift are removed.
Of course, this is just true for a standard saga. Perhaps, your storyguide has decided for some reason that Faeries are treated hostilely in his saga. Which is fine. However, if you did not expect this then maybe you can discuss with him whether you can swap Strong Sidhe Faerie Blood with another Virtue, whose effect you both agree on. Afterall, it's frustrating and not much fun if your character doesn't do what you expect.