I have had a problem like that with other products. What I ended up doing was to type it in.
You might check and make sure and numbers lock isn't on, and that caps are not locked in.
You could also request a registration number look-up just to re-verify that you are using the right number. It has been a long time since I registered something.... but when I did I remember I was trying the wrong number for some reason.
If nothing else works you can contact sales@stardock.com. They are friendly folk.
(Tools > Settings > Internet)
You should have no problems copying/pasting serial numbers. Exactly what error message are you getting?If you continue to have problems, please email me directly with your account details and DX3 serial number. I will try it here and see what is wrong.