Make sure you’ve installed MAS 2.33 or later with your Digital Performer. MAS (MOTU Audio System) is a free download on motu.com
Also make sure the plugsound are in the MAS plug-ins folder (located here: System folder -> extension folder -> MOTU folder -> Plug-ins folder), and not in the application’s plug-in folder (that one is for Premiere and MIDI plug-ins).