How to debug a plugin with Xdebug?

I was able to install and setup quite easily NetBeans 6.9.1 and Xdebug on my local environment, based on PHP 5.3.0 (XAMPP).

My problem now is the following: if I put a breakpoint on WordPress’s index.php or other WordPress core PHP files, NetBeans correctly stop at desired position. If instead I put a breakpoint on a plugin file, an trigger the code execution NetBeans does not stop there.

Do you known how can be solved?

UPDATE: More info about what I am trying to debug

Plugin is called Vanilla Forums and has the following file structure:

- plugins
  - vanilla-forums
    + assets
    + nbproject
    + templates
      admin.php
      embed.php
      functions.php
      hooks.php
      plugin.php
      sso.php
      widgets.php

I set the breakpoint inside a function named vf_validate_options defined in functions.php. I am sure that this function is executed, but NetBeans does not break there.

There is an easy solution with a Firebox extension Xdebug Helper by Brian Gilbert. This sets cookies for the xdebug session which allows you to use xdebug within your WordPress plugins.

There are also extensions for Chrome, Safari and Opera. The links can be found on the xdebug website