On the main search page ( http://musicbrainz.org/search ) under "other lookups" there is a box to enter catalog numbers for a catno search.

For this type of search, you have to enter the catalog number without separators. That is, searching for "CRD3443" works. Searching for "CRD 3443" or "CRD-3443" does not work.

From looking around the old issues with searching by catalog number, I thought that the system was supposed to automatically delete separators such as spaces or dashes from the catno search term.