One design issue for such a page is that you can register for the download notifications without having a TPU forums account.
In other similar situations I know of, notification subscriptions are tied to the user account i.e. registration is required. So, my suggestion would be to do this and send a warning email to all current "subscribers" that non-registered users will stop getting notifications, unless they create an account.
Selaya's suggestions could work I guess, but personally I strongly dislike using email in such an utilitarian way (extra clutter). Email to log in, email to order pizza, email to access this and that... you get the idea.
update: It's usually like this: there's a link somewhere in the page (often top-right, close to the username/pic) that leads to the subscriptions page: a simple list with all subscribed software. Next to each software title, there's optionally a "delete/remove" button which removes the related subscription.
In the separate software pages, the subscribe button can act as a toggle. Press to subscribe, press again to unsubscribe. In any case, it would help if in the aforementioned list, each software title listed is linked directly to its page, so we can quickly unsubscribe in case the above "delete/remove" button is not implemented.