Huh? All Macs require USB keyboards. If you have a PS2 keyboard and the needed adapter to connect it to a USB port on your Mac, you should be set. You won't be able to power on the Mac from your keyboard though - for that you'll need to use the power button on the Mac itself... assuming you also have a USB mouse, or a PS2 mouse and a second adapter.
We're assuming you got a Mac Mini, as that is the only Mac which doesn't have a keyboard or mouse included.
If you don't have a USB keyboard or mouse around, I suppose the only solution is go out and buy one. You can use any keyboards or mice with Apple computers, although it'll be easiest if you got an Apple keyboard.
You ordered the Mini right? Thats the only Mac that doesn't come with a keyboard. Your best bet is to pick up a cheap USB keyboard, there like $10 nowdays.