You need to make sure SNMP is operating correctly before you configure MRTG. What's a good basic configuration, and how do you test it?
Start it manually with this command:
# /etc/init.d/snmpd start
To have it start automatically at boot, use chkconfig:
# chkconfig snmpd on
# chkconfig --list snmpd
snmpd 0:off 1:off 2:on 3:on 4:on 5:on 6:off
snmpd is controlled via the usual startup scripts:
/etc/init.d/snmpd {start|stop|status|restart|condrestart|reload}
Now, you can go back to the previous recipe, and follow the steps there.
This is a bare bones SNMP configuration that allows you to get MRTG up and running with a minimum of fuss, and to reduce the number of potential complications. See the Discussion in Recipe 14.2 for information on configuration options.
Net-SNMP: http://net-snmp.sourceforge.net
man snmpd.conf