This one may require a bit of work, if you don't have both characters unlocked.
Unlocking M.O.D.O.K. - Complete the main story mission High-Noon Saloon.
Unlocking M.O.D.A.M. - You'll find her side mission in Hala. Her location is easily spotted in the image below. Her mission is simple, just follow her and beat up a few AIM agents. She will cost 500,000 studs.
Edit: Worked fine for me without playing a level, but if you're having trouble try MushaConvoy's suggestion.
per MushaConvoy; "For me, it didn't work in the main game, had to choose MODOK and MODAM within Welcome to the Jungle before it popped."