Adobe Premiere Elements 10: File > Export is greyed out
The Export function was removed a couple of generations ago but it is still in the file menu as its used for a couple of things such as tiles.

All video is output using the "Share" button.

If you want some extra information on this matter here is an answer from Adobe:
http://helpx.adobe.com/premiere-elements/kb/export-option-unavailable-premiere-elements.html

Hope that helps!