I don't believe you are able to edit SNMP within the vCenter plug-in. In order to enable SNMP services on the storage nodes you will need to go to your MVIP and logon. If you don't know the MVIP IP address you should be able to find that in the plug-in under Element Configuration. Once you logon to the MVIP, you can select the cluster tab and you will find the SNMP services.
> we now want to manage the mNode via SNMP with our Fault-Management-System
Although this has been solved, I'd like to clarify that MVIP (mentioned in accepted solution) is the cluster Management IP which lives on the (storage) cluster, on the node with Master role, rather than mNode (the out-of-band management VM).