If you are hanging a sign, you'll need 1 or 2 anchor points. Pick up some large, plastic, screw type wall anchors. (They are the type used for drywall) and some eye-screws about the same size as the metal screws that come with the anchors.
1.) Screw the eye-screws into the plastic anchor.
2.) Screw the plastic anchor into the styrofoam.
3.) Unscrew the plastic anchor out and dip it in water, (because Gorilla Glue needs moisture to work)
4.) Put a drop of Gorilla Glue in the hole and,
5.) Screw the anchor back into the into the hole.
6.) Wait until the glue hardens then hang with fishing line or whatever.