The message there is incorrect for Mac OS X; cabal installs executables somewhere under ~/Library/Haskell, and (inconsistently and incoherently) sometimes in per-package bin directories under that.  This is a PITA that I don't know how to work around properly (just setting the bin directory ~/.cabal/config still seems to keep the occasional per-package bin directory silliness).

Expect to have to "find ~/Library/Haskell -name bin -type d" periodically to figure out where cabal put stuff, and symlink it all to ~/bin or whatever.  :/