Installing the Nagios package on Ubuntu causes some problems when working with the documentation. However, there some simple fixes to get things working.
First, know that all of the config files are located in /etc/nagios3/. This differs greatly from the standard location of /usr/local/nagios/etc/. You’ll also have to create the objects/ directory in there.
Second, the default templates are missing, but you can copy them from the Core package found here. Grab the definitions files from sample-config/template-objects/, remove the .in extension, and place them in /etc/nagios3/objects.
Third, you’ll need to comment out the duplicate definitions, such as generic-host and generic-service.
After all this, you should be able to get things working with the documentation.