I would use the remote control, but let's see if we can get you through this.
In the manual on page 37, it discusses setting up the input mode for the tuner. You can select "Analog" or "EXT. IN". I bet that you want "Analog," because "EXT. IN" refers to the multi-channel inputs (I believe). Try setting that to "Analog" for the tuner and see if that works for you.