You can request for the serial number either from the request a serial number web page.
If you don't find your serial number on this site look at the download page and see if it is an option there.

When you request a serial number the number is sent to the email address you gave us when you registered (you can change it in your Profile Settings if you need to). If you didn't receive the serial number, please make sure that it is not in your spam inbox because some spam filters may qualify the message as spam.

If you are still unable to obtain the serial number, please send your username and the name of the program you are trying to install to the Nokia Developer Website and if possible they will create the serial number for you manually.