I'm having this issue too and it's getting frustrating. Tried adding DTWID and it refuses to load, or even successfully inject into the game with DevMenu.
Yeah, I personally wonder if it's a save thing - my reasoning for this is that we use save injection to play MM2 levels offline, and the add-ons don't seem to be account-based or anything like that.
If someone who had all the add-ons could upload their save data, that'd be pretty great - I don't...
The newest update for DOOM and DOOM II allow you to make a Bethesda account and download add-on campaign wads from their servers - the current available add-ons are SIGIL, No Rest for the Living, The Plutonia Experiment, and TNT: Evilution. Now, we've figured out things like injecting Mario...