Quote - "If the item has only MAT poses setup for P4, you'll need to edit them to replace the BUM references with one to the JPG file of the same name."

How do I do this?

Use a text editor to replace ".bum" with ".jpg"
Most MAT files are quite small so you should be able to use WordPad (or the Mac equivalent) without difficulty.
Work on a copy of the file until your sure it's working right and watch out for instances where the BUM file has actually been supplied (not encountered that often, but it will save the annoyance of having to go back and alter the file again).