Hi Don,
The word from Apple is the issue is with the WWDR Intermediate certificate, which expired on the 14th, preventing apps purchased from the Mac App Store from validating their receipts. They say a restart should be all that's needed, but we've heard from several folks, on a wide range of versions of OS X, that restarting hasn't helped. We don't see the issue here on any of our test Macs, making it difficult for us to troubleshoot, so we're recommending reinstalling the application (just the application, leaving the data in its document, elsewhere on the Mac, intact). Here are the full details:
http://www.splasmata.com/?p=3042. Can you try reinstalling? We're very curious to know how reinstalling from the Mac App Store works out, and whether the workaround of using the version from our website is necessary in your case. Thanks!