First program a frequency you want and then hit Mode until it shows 'CT' at the bottom of the screen. Now, you can either wait for a transmission and if a PL tone appears you can hit Enter to program it into the channel or if you know the tone already, you can just program it manually by pressing PGM then FUNC + Mode and use the arrows to scroll to the tone you want and press Enter.
For more info, you can consult the manual also.