Stencyl runs perfect on a Mac and is, especially when it comes to the Mac, the best choice (okay, it's my choice)
First, try to repair permissions and check rights: open DiscUtility, select your hard drive and press "Repair Disc". Don't worry, it doesn't harm anything.
If this hasn't work, open the Console (open Spotlight and enter Console, it should show you the app) and then start Stencyl. Watch the output for some Java and or Stencyl-related output. If there's something, copy it and post it here in the thread.
/dripple