Tapper, I believe you have already figured this out but I will reply for others that may have the same question.

The basic process is that you install the USPS Shipping Assistant software. This can be found at http://www.usps.com/shippingassistant.

After the USPS Shipping Assistant has been installed and registered, the UserID can be found in the “About” box, in parentheses to the right of the name of the registered user.