Market Press Error Message With Pay Pal Integration

I installed marketpress on my website:

http://shop.planet89.com

I set up an API signature with Pay Pal and put the information into marketpress. Now when I go through the check out process I get the following error message:

There was a problem connecting to PayPal to setup your purchase. Please try again.
10002 - Security error - Security header is not valid

What should I do?