Andy's answer is basically correct but I note that he said you should 'tune' the recorder. The signal coming into the recorder won't be a radio frequency signal (like the one which comes down your TV aerial). It will be 'composite video'. Your recorder (or, more probably, its handset) will probably have a button marked 'AV' alongside the normal channel buttons. It's this AV 'channel' which you should select when you want to record the signal from your Freeview box.
As Andy has said, you can't watch one digital channel while recording another. (With conventional, analogue, TV there's one TV tuner inside your TV set and another one inside your recorder. Using a Freeview box with an analogue VCR, or DVDR, means that you've only got one digital tuner, inside the Freeview box. Obviously, if you buy a new recorder, with a built-in Freeview tuner, this won't apply and you'll then be able to watch one digital channel while recording another).
Chris