if you have the new version of gta san andras (v2) you cant play mta cause only 1 is supported, in 2 you cant mod stuff
so there is a program wich makes your v2 ito v1 and then youre able to play the mta mod
http://talidan.littlewhitey.net/gtasapatch.exe (To downgrade from 2 to 1)
source gta-forum