Hello,
First off I want to say I love this application! I have used Toon Boom, After Effects, and Anime Studio, and I have to say Spriter by far is the most simple. Best 25$ I have ever spent in my entire life!
Now to the bug, on the Mac version when I try to activate a character map (by dragging it to the active column) is does let me until I click Merge Active to new -> cancel and then Bake Active to Entity -> cancel and then it will let me drag the map into the active column and works for the rest of the time the application is active.