I think you're easiest route is to get the check_snmp plugin installed. This is available as part of the standard plugin package but the installation routine will check to see whether you have the required NET-SNMP package installed first, and if not will skip install of that plugin, so in a nutshell all you need to do is
1) install the NET-SNMP package
2) re-install your nagios plugins
and then you should in theory have your check_snmp check ready to roll.
Find out all about NET-SNMP including download and how to install it from here - net-snmp.sourceforge.net/
Good luck and if you get stuck do come back and I'm sure someone will be able to offer some assistance.