For a cable box such as comcast/time warner you should not have to put in anything in 'start' action. Typically you would have the cable box set to wait until it gets 3 digits such as 0 0 8. If not, you may be required to press 'Enter' to complete the sequence such as 8 'Enter'. If this is the case, you would set the end action to the 'Enter' function.

Hope this clears things up. If you are using PEPv1, you can have a look at the help file (F1) and read up on the Channel Macro Wizard.