You have to install teTeX, a freeware TeX implementation available here. You also have to install a MacOS X front end. The premier front end teTeX app is TeXShop, which is also freeware and available on the same site. Just download the disk image file named ii2.dmg. Install and run i-Installer. i-Installer will download and install all files required for teTeX.
On there site there is also an equation editing utility, which is quite handy for really long and complicated equations with lots of special symbols and brackets. On XP TeXade made equation. Tex this is a beauty!