You need Account Services to move the 6600 to the customers account. You can call by phone as stated by Andy above or create a Non-Technical SR# in customers UserCenter. Key to registration is the MAC address that also should be on the invoice as on the product label(s).
Usually, the product is moved to the customers account that ordered it before shipping. If you know that the 6600 is in a different Account, then the admin of that account has to move it to your customers UserCenter as CP can not do that.
CCSE CCTE CCSM SMB Specialist