You can launch the OCaml toplevel like this:

$ ocaml

If you want to turn unexhaustive matches into errors, do:

$ ocaml -warn-error +8 

once in the toplevel, load the files like this:

# #use "cards.ml";;

Once the file is loaded, you should see the types and values available, you can invoke a function like this:

# card_value c;;
- : int = 9

If you do not want to install OCaml, you can use this website: tryOCaml.