How do I create an account with Wizards of the Coast if I have a very old DCI number without activation code?
The website http://wizards.custhelp.com/app/answers/detail/a_id/2301/=1

Actually answers this question, but not quite obvious.

It says numbers shorter than 10 digits have been converted and that you can receive your DCI number either at a local WPN store (where you have to ask) or via the contact form.