Just copy ATTACK.WAD and the other 19 WAD files (Canyon, Blacktwr, Virgil, etc etc) included with the Master Levels into the same subfolder "source_wads" as you have doom.wad, doom2.wad etc, and run WadSmoosh. The window that pops up should show "The Master Levels" in the episode list.
oh i see, i thought i had to put them in a folder, opps. thanks. also, is there only 20 files for master levels or 21? i saw teeth wad is suppose to be a two part one? or no?
Also, can you make one for wolfenstein 3d and its 3 expansion packs too?
TEETH.WAD contains two maps, so yes it's 21 maps across 20 files.
Wolfenstein 3D is totally different tech and totally different file formats, so that would be a whole other kind of project for someone else who knows about those.