How do you plan on using the Kindle? if you just want stock and want to connect to amazon, you can try updating software, if you want to use google apps, root or install a custom rom, you should try doing that first and not worry about registering.
Not having this device not sure but
It may have been rooted and stock was to reinstalled correctly, or just a software error
There is a software update 10.5.0 and should have installed automatically when connected to wifi (registered or not).
You can manually update, need a pc or mac
download
update-kindle-10.5.0_D026_5060020.bin verify 579.74 MB (607900588 bytes) md5:6a2dd7841355a7ed513607591a3499ec
place on Kindle sdcard/storage see
http://www.amazon.com/gp/help/customer/display.html?nodeId=201177720