I have several ESX 5.0 servers that are successfully polling and returning data from CPU/Storage/VMs, however the system memory is not getting polled/graphed. 

This is running the discovery process with debugging:

Memory : DEBUG: SNMP Auth options =  -v2c -c alpha
/usr/bin/snmpbulkwalk  -v2c -c alpha -OQUs  -m NETSWITCH-MIB -M /opt/observium/mibs/rfc:/opt/observium/mibs/net-snmp:/opt/observium/mibs/hp udp:flux:161 hpLocal
hpLocalMem = No Such Instance currently exists at this OID

DEBUG: SNMP Auth options =  -v2c -c alpha
/usr/bin/snmpbulkwalk  -v2c -c alpha -OQUs  -m NETSWITCH-MIB -M /opt/observium/mibs/rfc:/opt/observium/mibs/net-snmp:/opt/observium/mibs/hp udp:flux:161 hpGlobal
hpGlobalMem = No Such Instance currently exists at this OID