Paypal Not working

I signed up at Paypal, got all my API info, entered it into the appropriate boxes in MarketPress. I also got my USPS API info, installed and working. But when I do a test purchase, when it comes to payment, 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"

I should also note that on the payment page, there is nothing to select, (I only have PaypalExpressCheckout selected in the store payment setup page).

What does this error mean? I have sandbox selected in my payment setup also.