Nagios3 is in the repository for Ubuntu 8.10.
sudo aptitude install nagios3
Once it is installed run a pre-flight check to verify it is working correctly.
sudo nagios3 -v /etc/nagios3/nagios.cfg
By default it should run and you should be able to login to the web interface after you create the nagios user. Move into the /etc/nagios3 directory and create a user to access the web interface.
htpasswd -c htpasswd.users nagiosadmin
New password:
Re-type new password:
Adding password for user nagiosadmin
Change the user permission to htpasswd.users
sudo chown root:www-data htpasswd.users
sudo chmod 640 htpasswd.users
Now login to the web interface with http://SERVER_IP_ADDRESS/nagios3/
You will not have all of the rights to view everything but for the purposes of making sure it all works you will see it is all running. Now you need to configure it for your settings.