You're missing the point of an emulator. It let's one system (the one you're using) emulate another system (one you don't have/can't use). All the games you download for emulators are games for the system you don't have. If they don't work in the emulator, then it's usually not the game at fault, it's the emulator you're using that has the problem. Either try another emulator, or work out how to configure the one you're using so it does work correctly.
EDIT: Also, yes, you can play both PAL and NTSC games on the same emulator, provided it's a decent emulator like ePSXe - dunno what the mac equivalent of that is though, or if it's as good :p