I am not certain to the nature of this error, however occasionally when picking sub-dialogs such as the Load Curve function button in the Applied Force Dialog, the sub-dialog never shows. The application seems to be Sub-locked up. The problem is that the modal dialog for the curve is trapped in the background.
Here is what I did to get it back up front:
Pick the Simulation Application top bar, and then hit escape.
This may be addressed in the service pack, however I have not tested this in order to be certain.