In my old box [with Sonar 8.5], it's in ... \Program Files\Cakewalk\Shared DXi\PSYNII. You're looking for PSYN.chm; right next to the PSYN II.dll and Presets subfolder. Click on the PSYN II GUI to give it focus, and press F1 to bring it up.
There's a little bit more on PSYN
here.