I checked the "Add checkboxes to allow selection of multiple categories" box in the core Category list widget properties. However, that doesn't give me the "get selection" button necessary to actually search by multiple categories.

I thought maybe it was just the skin I was using (Glossyblue) but I tested it with other skins and had the same problem.

Secondarily (and a bit off topic), on the skin I am using (Glossyblue), the spacing on the search text and radio buttons are off. Instead of something like this:

( ) All Words ( ) Some Words
( ) Entire Phrase

It looks like:

         ( )      All Words             ( )
Some Words          ( )               Entire Phrase