[Haskell] Importing things via the FFI?

So I decided to pop into the FFI Report thing. I came up with this C file (which may or not actually be valid C; I'm not a C guy), called ffitest.h:

int add(Int x, Int y) { return x+y; }

And ffitest.hs:

import Foreign.C.Types

foreign import ccall "ffitest.h add" add :: CInt -> CInt -> CInt

Ye error message:

During interactive linking, GHCi couldn't find the following symbol: add This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. (blah blah blah)

Obviously I'm doing something wrong here, but what is it?