If you're having problems getting a WAD to work, try the following checklist:
[ ] The WAD file name must end in an uppercase ".WAD".
[ ] Your WAD file name is not "DOOM.WAD" or "DOOM2.WAD".
[ ] Do "Check Everything" from Hellmaker's Check menu.
[ ] Check that you have saved in the correct format – DOOM 1 for Ultimate DOOM.
[ ] Use MacBSP on the level (See the "Non-Graphic Structures" chapter for more information).
[ ] Check for Orphan vertices (shown as circles in Hellmaker's Map Mode) – they may cause problems in some cases.
[ ] Check for any linedefs that intersect (cross over one another) – they will cause problems.
[ ] Try launching by drag'n'drop rather than double-clicking – if you have multiple versions of Doom (including the Doom demo) the wrong one may be starting.