I do not have Words With Friends, but:
Somewhere in the WWF app ther should be a place to enter your FaceBook ID and password. It should take you to a screen where you authorise the app to work on FaceBook.
If you did this and it is still not working, then you need to go to Facebook and check you privacy setting to make sure you are allowing the app.
Hopefully someone who uses the app can give you more specific instructions.