If I have my project set up like this:

.
├── file.ml
├── plugins
│   └── puz
│       ├── puz_bin.ml
│       ├── puz_match.ml
│       ├── puz.ml
│       ├── puz_plugin.mllib
│       ├── puz_types.ml
│       └── puz_utils.ml
└── _tags

$ cat plugins/puz/puz_plugin.mllib
Puz Puz_bin Puz_match

This works:

$ ocamlbuild -use-ocamlfind plugins/puz/puz_plugin.cmxa
Finished, 22 targets (3 cached) in 00:00:03.

My _tags file:
------------------------------------------------
$ cat _tags
true: thread,debug
true: package(core_kernel)

"plugins/puz": include

<gui.*>: package(labltk)
<**/puz.*>: package(unix), package(str), package(core_kernel),
package(bitstring), package(mikmatch_pcre)
<**/*_bin.*>: package(bitstring.syntax), syntax(bitstring)
<**/*_match.*>: package(mikmatch_pcre), syntax(camlp4o)
------------------------------------------------

But I can't figure out how to use this library in my main program. This is
a minimal example of what I'm trying to do, not working of course:

$ cat file.ml
open Core_kernel.Std

let read fname =
  let data = In_channel.read_all fname in
  Puz_plugin.read data

$ ocamlbuild -use-ocamlfind file.native
+ ocamlfind ocamlc -c -g -thread -package core_kernel -I plugins/puz -o
file.cmo file.ml
File "file.ml", line 5, characters 2-17:
Error: Unbound module Puz_plugin

martin