You need OpenManage Agents to be installed and SNMP (not just Traps) enabled on each server you want to monitor. Also, you'll need the official and model specific MIB(s), which you copy to the (InstallDir)\Data\MIB-Directory (Restart all WhatsUp related services after the import).
Normally, I start by scanning the MIB using the SNMP MIB File Explorer, which is integrated in the WhatsUp web interface (Search by: MIB Module); As soon as you have found the SNMP "branch", where you assume finding the values you need, you select it and switch to the SNMP MIB Browser.
Here, you select your "reference device", the valid SNMP-Credentials and start the look up.
If you have further questions, just ask.