That option should just be plain, vanilla "Galaxy", not "Galaxy-theme".
When you open the archive, it contains a folder called "Galaxy" and all the theme files and folders are within that. It's that "Galaxy" folder that you should extract to your themes folder.
If you have used the "Extract" option that extracts the contents into a new folder named after the archive itself, then simply go to your themes folder, go inside the "Galaxy-theme" folder, right click the "Galaxy" folder, choose "Cut", go back up, into the themes folder, right click folder background and choose paste. Now delete the empty "Galaxy-theme" folder, set your theme to "Galaxy" in MC, and you should be all sorted.
-marko