Is Mapster in the same directory as your World Tour game? I believe Mapster only looks in the local directory (and DUKE3D.GRP within the local directory) for ART files.

I've got Mapster in my World Directory, and it loads the new ART without any issue.

There is however the issue where people could build maps with user art occupying textures #5120 through #6143, but instead of skipping to #6144 to start their new textures to avoid potential conflict with user-made add-ons, the World Tour developers put their new tiles within that range, so if you're trying to look at an old user add-on that that used that range, it might have blanks where the World Tour maps expect textures.