I will be posting a few OD+ mini tutorials on Thanksgiving weekend (which is when I have enough time to do it justice).
I will try to include as many procedures as possible, while staying in the "mini" format.
The "Load theme" button should give you a list of themes, not the empty text box. Uninstall/Reinstall should solve, as long as you save your current downloaded content to a temporary folder, and delete the OD+ program and cache folders before reinstalling.
Agree on the theme issue, best bet is to change the extension to ".zip" and manually place the files - my current mini tutorial should address placement.