How do I get ghci to see packages I installed from cabal?

You need

ghci -package such-and-such

And to double-check that such-and-such is truly visible to GHC, run ghc-pkg list | grep such-and-such.

ghc-pkg list on the command line will tell you what your installed packages are. The installed package might be hidden, in which case you can reveal it with ghc-pkg expose {pkg-id}.

Use cabal repl to ask cabal to open the GHCi interpreter with all the right settings for your project.