[Haskell-cafe] Cabal install problem

Lately whenever I try to install a cabal package it fails with the following error message: Could not find module `Control.Monad.State': Perhaps you haven't installed the profiling libraries for package `mtl-2.0.1.0'? Use -v to see a list of the files searched for. The strange thing is that I can load Control.Monad.State from ghci. How do I fix this -- i.e. where should I look?