If you go to
www.nuiaddon.com you should see a link at the top showing nUI+. Click on that and it will give you a newsletter sign up page. Fill that in and follow its instructions when it arrives. You should then in a short time get access to the download page here and able to install it.
The update service is for those that would like a copy of the addon automatically emailed to them as an extra option. You don't have to have that if you don't want to.