You need to click into the Support area for the Trial Pay section of offers, scroll down the list of all offers you have participated and select the Credit Card one.
Then submit your proof. usually the confirmation email, including the email header.
at the Bottom of every page in WGT there is a SUPPORT link for WGT this will offer a link to the 'CUSTOMER SUPPORT TEAM'
http://www.wgt.com/help-request.aspx
also, this is email address: members@worldgolftour.com
They are usually fairly prompt in reply however they can get busy so need to be more patient.