@Prajna Just want to confirm, after you put in the money to your account, did you actually go to the Addon pay to Buy the addon?
Getting Addon is a 2 steps process. First, load the amount. After amount is loaded, it will appear as "Available Fund" Second step is to actual buying of the addon
Do you mean you added 2 x 1GB add-ons - OR - that you're on the $35 plan which includes 2 GB of data?
I suspect the former.
See if your payment card was charged TWICE for $15.00, and also that the charges are showing under your transaction history on your self-serve account.
They'll either be there or in Available Funds (if you're card WAS charged twice).
It could simply be a matter of selecting and adding the 2nd 1GB add-on as outlined above if the funds might be displaying in Available Funds. A recommendation might be to let the 2nd add-on payment just 'rest' in the Available Funds (if they're there), and use them when you're close to consuming the 1st add-on.
@Prajna :Can you log into your self serve and post screenshot of it for us and edit out your personal information too? That way we all can help you see what there or not.