It turns out there was an 'ICMP Only' box in the node properties that was checked, and when this was unchecked, an SNMP configuration area appeared where the public string etc. could be configured. After configuring this, the volumes were visible through the resource list, and I was able to select the ones I needed.
I would strongly suggest not having the SNMP configuration area's visibility toggle on or off. The screen space saved is not worth the confusion it creates.