Since late last week, NZ has had three official languages - English (de facto, not de jure), Maaori, and now NZ Sign Language. Are there any other countries that have made their SL an offical language of the country?

Please, how does a language become "official" and what does the term imply?

the US doesn't have any official languages...there are some who would make it bi-lingual, (English and Spanish) and more who feel it should be English only...

citizenship does requires a basic knowledge of english.--but its written/reading english, not spoken english.
the league of women voters-a non partician group that have made it there business to distrubute voter information alway have info in a host of languages.

NYC has bi-lingual education programs, 90% of which are english/spanish, even though spanish speaker only make up something like 40% of the non english speakers.
the general held view is bi-lingual (english/spanish) classes present the opportunity for kids to grow up illiterate in 2 languages.

In NZ, Maaori's official status means that all government documentation is available in two languages (my passport for example is in both). All government departments have two names, and my favourite is the Inland Revenue Department Te Tari Taake.
It also means that the languages are sanctioned for use in court, meaning that the entire proceedings of a court can be conducted in that language. Apparently, this was a big part of the reason for making NZSL an official language. Also, in Parliament, an MP can speak in any of the official languages without being required to translate, as used to be the case before Maaori was granted official status.

Making sign-language a official language seems like a political move to garner favor with the populace - the "gee, how nice and caring is that party."

What are the basic rules that are followed to make a language official in NZ?

Here it is the greatest number of speakers using the language - thus English and French.

I have friends who have worked as NZSL interpreters for the Justice Dept. here, and the TV piece on the change indicated that the court situation was a major reason for the elevation in status. Clearly, the NZSL decision was a special case, and I'm not sure of all the reasons, but the other two are more "normal" choices - English by volume, and Maaori for reasons of historic significance.