I've bought recently an Ibanez rg1527, a 7 string model with floyd... but since I changed the string (two days ago) that goddamn D string just keeps popping up from the bridge... I'm sure I change string correctly because a friend showed me how (he uses floyd since 1 or 2 years ago), but it just keeps coming out!

Sorry for mistakes but I'm in a hurry, tomorrow evening I have my first show! Can someone help me??!!
If it's popping out, it's not in right. Check it really closely as you tighten, you should be able to see it pinching properly
Yeah, if its not holding the string tightly, either you aren't tightening enough or the string is not in the right place. Tempoe pretty much hit the nail on the head. If that isn't the problem, you may have a stripped thread or bridge defect somewhere, which would really suck.
What do you mean with "pinching"? Show some mercy, english isn't my main language

I think I do the same thing I've done with the other strings, and the little black lock doesn't seem to be broken...