Error Message on Store and Products pages

I'm getting the following error on my Store and Products pages. I've updated to newest version of marketpress e-commerce and Pro-Flexmarket. Please help me.

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/bikeworks/public_html/wp-content/themes/pro3/header.php:2) in /home/bikeworks/public_html/wp-content/plugins/marketpress/marketpress.php on line 989