Mac Users, good news. A Mac Users on the Battle.net Starcraft 2 Forum has discovered a work around for the Editor not working for custom maps.
What you need to do is to select the map you want to play; select map, right click, Open With, select Other..., search for the SC2Switcher.app, select Enable All Applications, select SC2Switcher, then click Open.
This will load the map so you can play a game with it. One problem however, there is no Save game in the Options so you will have to play the map all the way to the end without stopping to play it all. Not great I know but it's better than nothing. But I tried this way and it works, hope it works for you.