I encountered this problem when I was trying to test on my iPad in addition to my iPhone.
SOLUTION:
1. Go on developer.apple.com, register a new device, and add your new device's UDID.
2. Create an ad hoc provisioning profile, and select the new added device and download the profile, and double click it.
3. Open Stencyl -> IOS certificatates, and select the new provisioning profile.
4. Open finder, go to ~/Library/MobileDevice/Provisioning Profiles/ and delete all profiles here.
5. Open XCode -> Window -> Preferences, and download certificates and it will refresh all your profiles!
6. You will now be able to test on your new device!
