As far as I know there's no need of additional plugins, as it is a natural function. And OBH, in my experience (only with a NAS, not other STBs) is much comfortable in mounting. What OBH version do you have?
Now, the problem is: does VU+ at least find the network drive or not? Because, if it doesn't find, the issue is in the DM not in OBH, I think and you must learn how to share you DM's HDD.
Instead, if OBH finds it but doesn't mount, you should try different protocols, enabling them under DM.
For my NAS, under OBH, I've always used CIFS with mount at boot.