Have you received any issues of the physical magazine? If so, there's a subscription number on there. You have to enter that and your last name at the @Gamer site to log in to get the digital magazine I would assume.
Just signed up near the end of last month and the csr set it for digital so I have never received a print issue. I checked the receipt but there isn't an account number there either. I was hoping someone who has digital may have gotten an email about accessing the April issue (not sure if digital is available yet). I decided to just email the customer service address and see what they would say. Thanks for the thoughts though.