OpenRA

I've been playing OpenRA with a group of friends quite often lately, but after quite a few matches it became apparent that there was a slight dearth of maps.
Digging around, I came across the editor, and discovered it was able to import maps from the original Red Alert, which was great. Red Alert, with Counterstrike and Aftermath, had hundreds of maps!

Unfortunately, after selecting the import function and navigating to my Red Alert folder, nothing was found, besides one of the game's .ini files.

I currently have the First Decade version of the game installed; could this be the source of my problems? I have an original copy of Red Alert on hand if that will solve matters, regardless, I'd like to know why I can't find what I'm looking for, if there is indeed a discernible reason.
Thank you in advance for your assistance.

EDIT: I suppose it's worth adding that I'm running the Windows version of OpenRA on Windows 7 x64.

The editor won't look inside the mix files for maps, you must first extract the map inis (or ini+bin for cnc) and point the editor at those.