I haven't used OCamlMakefile, but Brian Zimmer wanted to, and figured out the following invocation to use it. Your Makefile should contain something like this:

OCAMLMAKEFILE := /opt/local/godi/share/OCamlMakefile

PACKS   = pa_arg # other syntax extensions here ...
PP      = camlp4find $(PACKS)
export PP


and your sourcefile (that uses pa_arg) should begin with

(*pp $PP *)

Brian warns that he couldn't get it to work with the version of OCamlMakefile available from godi (as of 2007-02-27), but it worked with the later version (6.24.5) available at OCamlMakefile's site.