Which version of Cakewalk are you using? I'd recommend using one of the newer versions such as Sonar 3 or 4.You want to use GPO as either a DXi or VSTi plugin. If you are going to be inserting tempo changes during the song, then I recommend to use the VST version.You'll need to close out of Cakewalk Sonar to run the VST Adapter. This is included with Sonar 3 or 4. See the manual on how to add a VST instrument.Open up Sonar after the VST Adapter finishes.Click on Insert, DXI and scroll down to GPO. You'll need to click on Insert, DXI, VST if you installed as a VST instrument. This will also add an audio track for the output for the inserted instrument.You'll then need to add a new MIDI track.
You'll need to set the output to the newly added GPO plugin and select a MIDI channel. Add an instrument into the GPO plugin and select the channel you want it on.
By default the first slot is Channel 1, 2nd slot is Channel 2, etc.You should now be hearing the instrument you added.-Another option for doing sheet music is to use the included Overture SE program. This is a dedicated music notation program and will get better results for printed music than Cakewalk.There are tutorials on the website on how to setup Overture, GPO Studio and GPO player.