Go to the main menu, in the centre should be a source called PC (it looks almost like a TV channel: a square), if you select this it should work.
Alternatively, you can specify the channel by scrolling all the way down, and in the bottom menu should be the name of the cable. I know that's not a good explanation, I'm going from memory!
If you can't, write to me again and I'll try to give you more specific instructions.
In any case, it took me a moment to find it too... that shows how good the interface is....