The error, "The application Digital Performer ( or AudioDesk) could not be found because an unexpected error has occurred." refers to the absence of a shared library in the System Extentions folder. This could be caused by using a system software earlier than 7.5.3 on a Power PC Mac. If you are using 7.5.3 or later, the error message means that the SOMobjects or Shared Library Manager extension is not active. Try restarting the Mac with the Base OS Extensions active in addition to the following:

FreeMIDI System Extension
FreeMIDI Power Plug
MOTU USB Driver (if you have a MOTU USB interface)
Shared Library Manager
Shared Library Manager PPC
SOMobjects
Sound Manager
Serial Tool and/or Serial (Built-in) should be active as well.