Error messages on Checkout page

Hi, I am only just setting up my site so I don't have any products listed in my store as yet, however on checking my pages on the front end I am getting the following errors on the checkout page:

Top of page error message:
Warning: session_start() [function.session-start]: open(/tmp/sess_km21vaelup2u16i7r6nqungb95, O_RDWR) failed: Permission denied (13) in /home2/myshoppi/public_html/wp-content/plugins/marketpress/marketpress.php on line 974

Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent (output started at /home2/myshoppi/public_html/wp-content/plugins/marketpress/marketpress.php:974) in /home2/myshoppi/public_html/wp-content/plugins/marketpress/marketpress.php on line 974

Bottom of page error message:
Warning: Unknown: open(/tmp/sess_km21vaelup2u16i7r6nqungb95, O_RDWR) failed: Permission denied (13) in Unknown on line 0
Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct () in Unknown on line 0

I am using the Flex Market theme from marketpressthemes.com. This is a link to my sites checkout page: http://www.myshoppingaustralia.com.au/store/shopping-cart/

Could you please advise how I can fix this. Thank you