It is possible that the plug-in has not been scanned correctly.
To force a re-scan of the plug-in, do the following:
Trash the "Audio Unit Info Cache" in
HD/Users/<Your Username>/Library/Preferences/Digital Performer
Restart Digital Performer. It will re-scan all the available plug-ins, then the plug-in should now show up in the list.
Go to the Digital Performer menu > Preferences.
In the Preferences window, select 'Audio Plug-Ins', find the plug-in in the list and click on the "Reexamine button". The plug-in should now show up in the list.