dd24 wants some written/faxed statement of the domain owner/admin to authorize a transfer to another registrar. I usually send them a fax in advance stating domain and receiving registrar and then tell the receiving registrar to start the transfer.
(BTW, AFAIK, in general you cannot transfer domains TO another registrar, you can only request them to be transferred FROM another registrar.)
To avoid the registrar transfer hassle, I use to advice my buyers to open a (free) account at dd24 and then push the domain (quick & easy).