Gallery2 checkout module problem

Hi, My name is Cristiano, from Brazil, and I'm running the checkout module 0.4.3 for gallery2 with the wordpress and WPG2 (plugin for wordpress)...

When I use the url for Gallery2, evering works well. But when I use the url from wordpress, I can see pictures and add to cart, but if I press "continue shopping button" I receive an error

" Forbidden
You don't have permission to access / on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request. "

Any help is welcome!

Ps: excuse-me for some errors, my english is not very good ;)

Gallery version: 2.3
Module / Theme name & version: checkout module 0.4.3
PHP 5.1.34
Webserver (e.g. Apache 1.3.33):
Database 5.0.45
Operating system: Linux