Using XPI to manipulate pref.js

I wish to change values in pref.js using a script. To make it simple,
I like to pack it in XPI, than let people install it.

I've read "How do I retrieve and modify Mozilla preferences?" article
at XULplanet (http://www.xulplanet.com/tutorials/xulqa/q_prefs.html),
but can't make that script work from inside install.js, without
getting errors.

Is someone here done this in the past and can help? Thanks in advance.