To add funds to the eshop, scroll to the left until you see the button with a credit card on it. I don't know what option you're using to add funds, but the credit card option has the MasterCard and Visa logo on it.
If you already have the fund added, go to the Animal Crossing game, click on the game (green background, white leaf) and it should take you to a page where it shows the logo again with a button underneath (I believe it is an orange button). The button underneath is the one you hit to buy the game.
I'm pretty sure there's another button that has to be hit after that.
