The error you are getting is:
You do not have permissions to make this API call
You need to have API access with credentials to make use of Express Checkout. According to the information that I posted from PayPal there is no charge for this. This is something that you should check into with PayPal. You do not need to sign up for Payments Pro or Direct Pay, you only need Express Checkout.
In the admin area if you are prompted to login it is because you are no longer logged in, this can occur if the session expires.