How do i install 3rd party apps with USB?

I have a T-mobile Mytouch 3G Slide, I just bought it and i don't have the android marker available. I want to install apps but all i have is the USB cord. I bought it from a pawn shop because it wad cheap. How do I install 3rd party apps without Wifi and without the android market?? PS: I Have a mac not a PC so sync and apps installer won't work.