The steps in the thread pointed to by Fionn should work...

Also, remember that once you've done the pairing _once_ it is not necessary to do it again!! To use the keyboard, once it has been paired, you just open it and hit a key (I usually hit the up arrow and/or down arrow) and it should automatically link up with the n800. If you have the bluetooth status icon on in the status bar, you'll see it light up blue when the keyboard is connected.