I know this question is over 2 years old, but I wanted to post the real solution in case anyone else ever comes across this problem.
There is a key on all keyboards called F-lock. It changes the actions of the function keys. BUT, if immediately upon power-up of the pc, you press this key right away, your wireless keyboard will function.
You've gotta be quick. I usually watch the power light on the monitor, and the second it goes blue, I hit the F-lock and then whatever key you need to in order to access your bios. You don't have to hold it down or press it at the same time.
The key probably isn't labeled F-lock anymore, that's just what it was called when it was first introduced. It can also be an "Fn" key, a "Function" key, etc. To track down where yours is, look at the 2nd set of words on the function keys. If it's a symbol or color, look around your keyboard for a similar symbol or color on one of the keys, and that will be it.
With a lot of support and help from those on this forum, I have finally finished my PC build:)
My next problem is Booting System for First Time. Is it even possible to do this with a wireless keyboard and mouse? I imagine the USB port has to be recognised before I can connect the USB device that came with the keyboard? There are no cable connections on Keyboard and dont want to buy another one just to boot up the system!