I am trying to find a download for ProMenu but there doesn't appear to be any search option for the downloads section or marketplace, whatever it's being called at the moment. 

Where should I be looking? 

You can search downloads by entering what your searching for in the search bar, and changing the "Everything" option to "Files"

http://community.invisionpower.com/search/?term=promenu&type=downloads_file

Or click on downloads on the main menu.

Clicking on downloads on the main menu only takes you to the portal for that area. If you want to search for something in it there really should be a search menu item that corresponds with that name. Having a place called "Downloads" and a search option called "Files" is pretty backwards if you ask me. 

It's just language bits not yet synchronized - everything is still in active dev, give it some time. Hell, Nexus abounds but it's Commerce now. They'll make a pass one of these days, probably right before release when they're all liquored up