You have probably got the TV tuner set to 'analogue'. For digital / Freeview press 'Source', then from the drop-down menu select 'DTV / USB' and press 'O.K.'
You can then press 'menu' and select Setup / Installation / Automatic Channel Scan.
I see you posted this query in January, so you've probably got it sorted out by now!
Ed Ward, April 2011