Send me a mail (info at nabocorp.com) and I'll give you the details. If you have it please enclose the paypal mail you got when you donated otherwise I will try to find it.
Does the registration include the right to updates or upgrades? Minor and patch updates (4.x.y) or also upgrades to new major releases (5.x.y., 6.x.y, ...)?
I suggest you update the license text to state clearly for which versions a serial number obtained through registration will be valid.