Firstly-
to the site.
Secondly-I've moved this into the Cable TV Discussion forum.
Thirdly-what do you mean you've "bought" an STB?
You clearly haven't bought it from ntl as they don't sell them-they only supply them with their DTV packages, but the box need to be registered against a live ntl DTV account (which I'm guessing you don't have?)
[Edit]-Simple answer is to phone up ntl & ask them to upgrade you from analogue to digital.
Full details here:
http://www.home.ntl.com/icat/television