okie dokey, here goes.
by the way, there is no mac specific version of ANYTHING for unreal tournament, but you CAN download & use the windows stuff.
doing this depends on what you are using. If you have Unreal, there is a UMOD which you can download. To install the UMOD, GT interactive has a UMOD installer you simply place into the folder with unreal, and then drop the UMOD file onto the installer. Thats it.
for UT, it's a bit trickier. There isnt a UMOD, only a .zip file. Download it and decompress it using stuffit expander or zipit. If you don't have one of those, get it. Both come in handy since once in a while one will decompress a UT related file (such as a UMOD) as a text file, which crashes the UMOD installer for UT (it's different, and can be found either on the web or on the CD UT came with). Anyway, decompress it and place everything in it's appropriate place. Anything ending with a .int or a .u goes in the system folder (the one in the UT folder, not on your hard drive!). For the sake of being helpful should you download maps and other stuff, .utx goes in textures, .unr goes in maps, and .uax goes in sounds. Then crank up UT and have fun.
see my response at
http://dynamic.gamespy.com/~unreal/modcentral/html/Forum7/HTML/000885.html
for whether to use it on the demo or the full version. As for whether it works on your computer, if you can run unreal (which you should be able to), then you can run inf. It's just a matter of whether you have enough VRAM for texture swapping (should) and enough normal ram (be sure to use virtual memory).