Hello.
Basically, we have a number of VMs and these require connectivity to one or more LUNs. Are RDMs required?
Yes, if you need direct LUN access then you can use RDMs to accomplish this.
If so, how does one connected a VM to the RDM?
Basically you create the LUN and present it to your host(s) the same way you would a VMFS volume. Then you add it to a virtual machine, just like you would add a VMDK. You just choose the RDM option instead. The full information on RDMs and connecting to them can be found in the ESX Configuration Guide.
Good Luck!