The account is through your cable TV or satellite TV provider. It is not actually and HBO account. If you subscribe to HBO from your provider, then you are eligible to watch HBO content through the app. I assume that you have used the app before and you know this, but your statement does leave this open to interpretation and I want to make sure we are on the same page. Actually, I don't think that I have ever seen a sign up page for HBO GO since you have to subscribe and sign in, in the app settings.
If you know all of that and you have used the app before, try closing the app completely and reset your iPad.
In order to close apps in iOS 7, you have to drag the app up from the multitasking display. Double tap the home button and you will see apps lined up going left to right across the screen. Swipe to get to the app that you want to close and then swipe "up" on the app preview thumbnail to close it.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.