it is impossible for you to have San Andreas Multiplayer on a
Mac for two reasons: #1. Grand Theft Auto: San Andreas only works
on computers running Windows 2000, Windows XP, or Windows Vista.
#2. San Andreas Multiplayer only works on computers that have Grand
Theft Auto: San Andreas installed, so since GTA:SA is not
Mac-compatable SA:MP isn't either.
Edit: there is actually a mac version of GTA San Andreas, but
the SA:MP client is only compatible with windows