That check probably doesn't support your device. Try doing and snmpwalk on the device and searching for memory values. This will give you the OID you can use with check_snmp which should be included by default with your nagios-plugins.
snmpwalk -c public -v 2c device_ip
search output for memory
check_snmp -H device_ip -o id from above -w range -c range -C public -m mibs
Your community may not be public but you should have gotten it from the other SNMP command you ran. You might also need to specify the MIB file when doing the nagios snmp check.