Should be a banner at the top of the app that says, "current subscribers, click here" or something to that effect. Once you sign up with your address or subscription number from the label on your magazine, it should (if it works) create an account for you, and then you're supposed to be able to use that login info to log into the app. Sadly, it's not working for me and some others.
Maybe I'm just lucky, but I had no problem in getting set up as a current subscriber. It took a while, though, and I thought a couple of times that the download had actually stalled. Fortunately, I got distracted from the whirly icon and when I came back there was the sign-up form waiting to be filled out. It went smoothly enough after that point.
ugh, i'm having the same problem. i found teh "already a subscriber" button, i enter my email and password... but then nothing. cannot get in. tried more times than I care to admit at this point. Awful ap!