using opam ?

Dear list

I just went through the OPAM web page.
http://opam.ocaml.org/
and I have a very stupid question.

It says it is a package manager for ocaml.
I assume it means that it can do the following:

I want to install a library foo -->
1) I type opam install foo
2) and then I can use foo in my ocaml programs ?

The stupid thing is that I don't know how to do 2). The packages seem to 
be installed in
$HOME/.opam/system/lib, whereas I have a preexisting ocaml install (ubuntu).
Do it mean that I need to include this DIR in my Makefile ?
Of course I can do it, but I see no reference to this in the OPAM FAQ, 
and it seems not very user friendly to me, so I must be missing something.
I would expect a package manager that gives me immediately usable 
libraries like gem for ruby, but maybe this is not the goal ?

San