Also, you need to have imported the StackSet stuff properly:
import qualified XMonad.StackSet as W
The StackSet type is heavily parameterized because sjanssen ran the XMonad.StackSet module through a code verifier which couldn't handle the X11 types, so substituted basic types for them instead. http://hackage.haskell.org/package/xmonad-0.12/docs/XMonad-Core.html#t:WindowSet is the actual type of what we normally refer to as the StackSet.