PRPtoOBJ is packaged with the HSLibPlasma. It can be found in the a download link here:
libHSPlasma You'll want either Plasma_Win32 or Plasma_Win64 depending on your operating system.
However I wouldn't use PrptoObj myself, only because it only allows the geometry to be saved, nothing else. For importing a full age it probably wont help all that much. Let me check Dustin's scripts and I'll see what's going on with his importer that it's throwing up these errors.
EDIT: Okay, after looking at your error, and the scripts, it seems this is a problem with Drizzle, not the importer itself. Understand that some information, regarding things like physicals, are lost when converting from MOUL or Myst V to the POTS engine, which is Drizzle's main function. It's unfortunate, but you're going to run into errors with some physicals, as they're simply not there within the files after conversion. The references may still exist, which is what the PRP Importer looks for, it checks for these references, then goes to that part of the PRP file and pulls the infromation it needs from that class. However if the reference is to a null class (IE Drizzle couldn't convert the class, so it left the reference to it on the object, but nothing else) then the importer will throw up the error that it can'd find something, because, well it's not there in the file.
You can try the original importer scripts here:
PRPImporter But chances are you'll run into the same error, as it's not a problem within the importer itself.