[CentOS] yum install nagios, adding dag repository

Hi,

I'm running an Opteron 64 bit server, and in an effort to re-join it to my
nagios monitoring, I have tried the install nagios via Yum.

When I type:

yum install nagios, or nagios-nrpe, or nagios-plugins, I get:

"Nothing to do". as output.

In wanting to add the DAG repository to my machine, the documentation i've
come across seems vague.

Can someone advise me on either of these issues?

I have come across the following HOWTO, but it kind of jumps around a bit,
and the if/else logic is too hard to decipher.

http://www.maxsworld.org/?page_id=115

Any help is most appreciated...

-karlski