Help! Blank Page & PHP Errors when trying to Checkout or Add to Cart!

Hello,

I am trying to setup and install Google Checkout mod on my ZenCart 1.3.8a website.

After installing the mod step by step as per the instructions I went to try using it, and I am getting blank pages when I click on "Add to Cart" or "Checkout".

Here are the PHP Errors showing in the error log:

[02-Aug-2008 10:04:35] PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/content/n/s/x/nsxgaming/html/shop/includes/configure.php:137) in /home/content/n/s/x/nsxgaming/html/shop/includes/functions/sessions.php on line 108

[02-Aug-2008 10:04:35] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/content/n/s/x/nsxgaming/html/shop/includes/configure.php:137) in /home/content/n/s/x/nsxgaming/html/shop/includes/init_includes/init_templates.php on line 78

[02-Aug-2008 10:04:35] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/content/n/s/x/nsxgaming/html/shop/includes/configure.php:137) in /home/content/n/s/x/nsxgaming/html/shop/includes/functions/functions_general.php on line 45

Please help, I have no idea how to fix this!

http://www.nsxgaming.com/shop