David, Am I correct in assuming you already have as many managed devices as your license in HiveManager allows? In other words, you have a 10-device license and are trying to add an 11th device? If so, you will need to purchase additional licenses. Your Aerohive representative or reseller should be able to assist you with that.
If you have not exhausted your licenses yet and are just trying to add a new device, I think I can help you. Navigate to the Monitor tab, select Device Inventory then Add/Import, and enter the device serial number. Now plug it into your network and wait about 10 minutes. It should boot up, phone home to our Redirector, be told which VHM is yours, and appear in the list.