First off, what exactly
isn't happening for you when you try it? I might be able to help you better if you mention exactly what's going on.
But if I'm thinking it's this... did you click on the box itself and choose an animation under one of the "Column" pie menus, then choose the "Overlay" option when the box pops up? Those are the only animations that are available to be used as overlays on sims while they do other poses/interactions. (you might also want to set the amount of loops that animation does to about '100') Once you get them to do the overlay, have them do any sort of other interaction or pose of your choice, and they'll do it while parts of their upper body will also be doing the overlay animation. :angel: