how to register 2 letter domain?
You've got more chance of finding a blonde virgin than finding a two worded domain ;-)
unless you're talking about country codes, in which case you need to register your domain with a country code (there are probably some limitations.. like.. you have to find a server in that country to host your data)
Like, in your example, co.nz would be new zeland http://www.godaddy.com/domains/searchresults.aspx?ci=54814
(assuming their links are persisten)
Otherwise, check out godaddy for more info.
Due to the popularity of owning a domain, its becoming impossible to find a domain for your uses... so its rare to find anything 2 lettered...
I would assume that vizz is asking about regiserting in one of these domains, not how to register a two letter domain...
I learned something new, that co.cc is a free domain holder... I guess I didn't understand the question.