I wonder if the folks reporting non-operational add-ons have tried again since the .dll update posted shortly after version 2.1 was released? I know some items (PMDG) simply will not work, on purpose, but many of the others will work with the updated .dll's.
If you haven't got the updated .dll's after having upgraded to version 2.1, visit here: http://www.prepar3d.com/forum-5/?mingleforumaction=viewtopic&t=5696