There is no ' add to Norton Management' link.
The Norton Mangement feature is tied to the Norton account that the Product Key was first registered to. As long as all PCs had NIS installed and registered to the same Norton Account, you should be able to use Norton Management.
You first need to check that all PCs have the setting for Remote Management set to on in NIS. Click on Settings - General - Other Settings. Look for the Remote Management setting and turn it on. As the PCs sync to the Norton servers, you should start seeing all the PCs listed.
If you have different Norton Accounts for different keys, you may try contacting Norton Support via online chat and ask if they can transfer your keys to the one account. This may not be possible and you will have to wait until your products expire and then register the new keys to the one Norton Account.