If you run into an issue that prevents you from starting Downcast on your Mac. You can usually get past this by resetting Downcast's preferences. This must be done carefully, otherwise Mac OS will prevent Downcast from updating it's preferences until you restart your machine.
Make sure Downcast is closed. If it is running and not responding, force quit it by right clicking its icon and choosing Force Quit
Open the Terminal app
Type: defaults delete com.jamawkinaw.downcast.mac
If you aren't using the default location to store episodes, re-select your media library folder in Downcast's preferences.
If you were using Downcast's syncing options, you'll need to re-enable those options.