Here is an installation and setup guide. Actually it has some good information about installing source packages using build-essential and checkinstall so it has a wider range of use than just Newznab.
Newznab on Ubuntu 11.10
Here is a link to the official documentation:
Newznab Documentation
This is a link to a page containing torrent files to get the backfill nzb's
Newznab nzb imports
And also a page describing how to import nzb's without bloating the database
How to backfill newznab safely without bloating your database