Hmmm.. what brand make is your keyboard and mouse kit? I've never set up a system using wireless kb/ms, but I know that the Logitech Wave wireless kb/ms works in POST and BIOS.
You may need to enable "USB Keyboard Support" in the BIOS (if you can). Some boards ship with it off for some reason.
There is no reason why the wireless kb/m should not work in POST - the receiver does not require drivers, it simply creates a link between the keyboard and mouse - the signal entering the USB port is essentially the same as if you were typing/mousing with wired peripherals.
I would double check that your keyboard and mouse are connecting to the receiver properly. Maybe you could try it on another computer?