newbie... can't change/control patches on my Yamaha CP300

Sometimes, depending on the instrument, you need to select GM (or Yamaha's XG) mode first. I used to have a Yamaha synth that required this to be either selected from the synth's menu, or by a sysex message from the sequencer. It should be in the manual. Sometimes the full midi specs are in a separate document that can be downloaded from their site.