Note: If you do not see the movie export button as being active and you have build version 0.72b (build 284) currently on the "nextversion" branch please delete the contents of the following folder:
"<steam_library_folder>\FaceRig\Mod\VP\PC_DataCache\facerig\dx9_target_dx9\Interface\Scripts\GuiMainMenu\" .

Then restart Facerig.

This is needed only once and will be fixed in future updates.

Of course replace <steam_library_folder> with the folder where you have the Steam Library where FaceRig is installed.