try these troubleshoots.
1... uninstall Stencyl completly
2... Go to directory where stencyl was installed (for ex. c:/program files/etc...) and delete stencyl folder(if existed)
3... Download a free program called "CCLEANER" and install it
4... Their's Option in CCLEANER "Registry Cleaner" it will delete any left over Reg. Files by Stencyl.
5... Install Stencyl now
Hope this works for you..