Is this the app store version?

Okay, you can do this, but it's going to be more complicated than if you had the regular version. All file paths are different in the app store version, since apple requires that everything be contained within the app package itself for app store programs. Start by right-clicking the actual editor application>Show Package Contents>Contents>Application Data>Photoshop Elements>10.0>Photo Creations and you should see photo effects, which I presume is the folder you're looking for. If you want to install in the actions player, it would be  Contents>Application Data>Photoshop Elements>10.0>Locale>en_US (or your location)>Workflow Panels>Actions.

In either path, when you try to put the file into the folder you will get a message that it can't be modified. Click Authenticate and give it your OS X account password and you will be able to put the action into the folder.

Edit: Oh, and mediaDatabase.db3 is in the Locale >en_US (or equivalent) folder.

Do not rename mediadatabase, delete it. Did you put in just the .atn files (and any .xml and thumbnail files), and not the folder they were in?

Did you delete all copies of mediadatabase and give the program time to rebuild it when you restarted it?

No, sorry, I really don't know why it's not working for you. As far as I'm aware it doesn't work in PSE 9/app store, but should work in PSE 10/app store. I certainly would agree that if the ability to add stuff (of any sort, not just actions) to PSE is important to you it's a lot easier with other versions, even when the app store version does work correctly. Updating the raw converter is much more complicated with the app store version than with the others, for example.