DELL OpenManage 7.3 on ESXi5.1

I don't think you can. It says that the only SNMP support is for sending traps, not for querying.

This configuration is only for sending SNMP traps to OpenManage Essentials. SNMP is not used for discovery. WS-MAN is the only protocol ESXi 5 supports.

Your monitoring tool will need to speak WS-MAN - Dell OME is one such tool. I am too lazy to look up Nagios' support for WS-MAN right now.

The documentation for check_openmanage seems to agree with me.

As others have mentioned, SNMP hardware polling doesn't work with OMSA on ESXi so you can't use check_openmanage, but since your goal is hardware monitoring, you might look at check_esxi_hardware.py plugin (for Dell, HP, IBM). It works exactly as advertised on ESXi 5.1 with OMSA 7.3 and has very similar output to check_openmanage.