Every DivX device has a registration code that is used to register the device to your DivX Account. Each device brand and model may place it in a somewhat different place, depending on the device's menu structure and organization.
Generally speaking, you can find these in your Setup or Settings menu. Sometimes you have to look under a submenu, but the Registration code will be found under the menu called DivX VOD, or just DivX.
You may also want to check the manual for your device. There should be a section in the manual referring to DivX Certification and explaining exactly where to find the Registration Code.