mrtg snmp and Debian

I installed snmp snmpd mrtg and have read the documentation although I
have not been able to get snmp working. When I snmpwalk localhost
public I recieve an error.

snmpwalk localhost public
snmpwalk: No securityName specified


Basically I want to set up my debian box to do something as basic as
keeping track of my cpu utilization or network usage with mrtg.

I am a snmp newbie so that is what is making this more difficult.