If you mean a linked subscription (paper & Nook), you likely have to go through the publisher themselves -- it won't be an automatic download (you may even have to use a computer to fetch the issues and USB to load them onto the Nook).
You'll have to go to the magazine publisher's website and see if they offer free digital subscriptions for their magazines.
If so, they'll have some details on how to get it for Nook (if that is an option).
A few months ago I used my free 14 day trial for a magazine then I bought an actual print subscription. About a month ago I canceled the subscription and now I bought a new one. In short I was just wondering how do I subscribe as an active print subscriber w/out paying again?
On the magazine page on the BN site there should be an area that says subscription benefits. In that area you will see something like already a print subscriber with a link to further details. Not all magazines offer a digital subscription with the print version.