i enter the first option "have existing account"
I enter old account SN and PW
next screen ask for registration code...
The only thing I can think of is you didn't even register it to begin with.
That being said, simply look for your old 1.0 codes and put them in, and you're good to go.
FFXI character: Elathia ::F Tarutaru::87SCH/75RDM/70WHM/54BLM::Cerberus/Ragnarok/Bahamut::1/23/2004 - 3/25/2015 :: Retired
RDM First 75 Job :: RDM Maat victory: March 28, 2008 (1/3) :: San d'Oria R10 Long live King Destin :: Praise be to the late King Ranperre. FFXIV character: Selene Silverstorm :: F Lalafell :: WAR60/WHM60/BLM60 :: Ragnarok :: 9/2013 -
Patch note archives for FFXIV: http://na.finalfantasyxiv.com/lodestone/special/patchnote_log/