You have to use Facebook Key Hashes block and run your game on Android. Then look in the log viewer in Stencyl or run the DDMS from Stencyl Debug for something like this:
PXR : pRqGr00vyKadfeNtfCttxWWk8JflaJk=
The hash is this: pRqGr00vyKadfeNtfCttxWWk8JflaJk=
For you it will be different string of course.