If you can't see the Export button, you have probably hidden the left side panel. Press F7 (or Tab) to get it back.
F5 shows/hides the Module picker, F6 shows/hides the Film strip, F7 and F8 show/hide the left and right side panels, Tab shows/hides both side panels. You can also click the little gray arrows at the sides of the screen to show/hide panels, right click the arrows for options.
There is no Export button in Develop, but you can use keyboard shortcut (Ctrl+Shift+E) to export, or go File > Export.