When I converted my 5.01 games over to 5.02 game, I did not see the greyed-out "Add Task" bug. Then, for no reason I can discern, about 10 minutes ago, I did. This tickled my QA-Developer sensibilities, so I played with it. I found that if I went to the Complex Activity Task Type combo box and selected something (even if it was the same thing already selected), this would cause the "Add Task" button to come back to life. (Which surprised me, since I wouldn't've expected those two pieces of the interface to interact.) Then, if I go to the Create Task Task Type dropdown and select anything, the "Add Task" button greys out again.
Given the behavior I'm seeing, my first guess would be that the code is referencing the wrong take type dropdown in an event handler or two. If you other folks seeing the greyed out "Add Task" button could confirm that you can get it active again by fiddling with the Complex Activity Task Type Combo Box, I'd be happy to know I'm not seeing things.