PayPal error with marketpress

Hi guys,
I am a newbie here, I am just setting up my first site with wpmu plugins.
I am getting this error message.

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

When performing a test check out,

Any help would be appreciated, I am sure it is my error,
Regards Andy