Fundraising Plugin Error (CODE)520003

Hi, I've installed the Fundraising plugin (version 2.5) and created a Campaign. I want to use Simple Payments so I've entered in the e-mail address for my Paypal account, and in Paypal I've turned on IPNs and and pointed them to the web address given. But I am getting the following error message when trying to make a donation: "There was a problem connecting with the paypal gateway. (CODE)520003 Authentication failed. API credentials are incorrect." It is set to live mode. How do I fix this?