PMB is Sony Picture Motion Browser, a file management system and the equivalent of Adobe's Bridge and Elements Organizer.
A PMB is not a video or media file, and you can not use it in a Premiere Elements project. Sorry.
Welcome to the forum.
What you have is a database file, that is created and used in the Sony software to organize files.
You will need to Import the actual media files themselves, the ones that are cataloged in the PMB. Do you have those files, or just the PMB database file?
The PMB can only be opened in the Sony software, or similar, and will just be a listing of the files shown in it and possibly a cached thumbnail, but no actual media files.
Thanks for writing I did finally figure out how to get them over. I
won't use PMB anymore since I have Premiere.
Great news. PrE comes with its own organizer, though being a built-in, might not offer all the features that a stand-alone might.
Good luck, and glad that you were able to Import the AV files.