[CentOS] Adding Repositories and using yum to download packages

Hi

I am using CentOS 5.3 and after installing the OS i scrolled to the 
/etc/yum.repos.d directory and i could not find any repositories being 
present there. So i googled a bit on adding the repositories and found 
the link:

http://www.osresources.com/index.php?action=artikel&cat=11&id=6&artlang=en

then when i try to install the net-snmp package that i require to 
monitor my box i get the following error:

[root at sc-devld-1 etc]# yum install net-snmp
Loading "rhnplugin" plugin
Loading "installonlyn" plugin
Repository base is listed more than once in the configuration
Repository contrib is listed more than once in the configuration
Repository centosplus is listed more than once in the configuration
Repository addons is listed more than once in the configuration
Repository extras is listed more than once in the configuration
This system is not registered with RHN.
RHN support will be disabled.
Setting up Install Process
Setting up repositories
not using ftp, http[s], or file for repos, skipping - 5Client is not a 
valid release or hasnt been released yet
Error: Cannot find a valid baseurl for repo: update

Please let me know what needs to be done.

Thanks
Jatin