Well since Apple doesn't really take part in these discussions, they weren't really asking Apple were they. Honestly folks, if you can't be helpful, then just don't reply! If you don't know the answer, which is evident here, then just don't reply!
To answer your question, don't click the Add to existing plan button, choose the creat a new account button. It sounds counter-intuitive, but you're not really using your existing cellular account, you are creating a new cellular account for this particular device, even if you are only going to use that service for a month.
The whole process is confusing and convoluted, but I'm assuming at least in att's case it's to try to get you to add the recurring plan to your existing cellphone account so they can get that activation fee.
Sorry you got those other useless bs replies above, there really are people here that will help you!