Assigning a WAD to a default program.

When I try to set a default program for opening WAD files, like GZDoom or PRBoom, the program doesn't appear on the "Default programs" list even after I've selected it, as though it's refusing to acknowledge that the program even exists. The only programs it seems to accept are completely incompatible, like Adobe Reader or Doom64EX.

Does anyone know what's going on or what I can do?

Assuming you are running Windows (which btw?), where have you installed GZDoom and PRBoom? Generally speaking the behaviour you describe is the default for applications which have not been installed under Program Files (or the x86 variant on 64-bit editions). AFAIK neither of these applications come with an installer so unless you've manually put them in the "correct" place, you won't see them in that list unless the application itself registers it's support for these formats in the registry (I don't believe either do but I could be wrong).