You need to do a couple of things,
First open a tab and go to
https://developer.paypal.comThen open your site in another tab, go through the checkout, then when you get to the "Express Checkout Page" Login with your buyer account
Then confirm the payment, you will then be sent to the URL as specified in the "RETURN" variable in the "SETEXPRESSCHECKOUT" API Call.
That is where you do the "DoExpressCheckoutPayment" API Call.