If you are not sure whether your version of PSE for Mac is "retail" or "App Store", you can find it out as follows.

Go to "Adobe Photoshop Elements Editor > About Photoshop Elements" (screenshot 1).

In case the "About" screen reads "Adobe Photoshop Elements 10 Editor" (screenshot 2), your program has been downloaded from the Apple App Store.
The appropriate version of Elements+ can be purchased here.
The free demo is available here.

If it reads "Adobe Photoshop Elements 10" (without "Editor"; screenshot 3), the program has been downloaded from the Adobe site or acquired on a DVD at a retail store.
The matching version of Elements+ can be purchased here.
The free demo is available here.