Open the Connect Store in the Connect software, and create an account. It'll automatically register your PC to the account (takes a moment or two).
Once that's done choose the Account option at the upper right hand corner of the Connect Store window.
On the lower side of the left column in the window it brings up, select Manage Devices.
If your Reader isn't already plugged in, plug it in now. Once you see it in the list, you should also see an option to authorize it to the account, select that, and wait for it to do its thing.
Now back on the account page, you'll see on the right column at the bottom, "Transaction History" (or something close to that), select it, and you'll see your credit listed there.
Now just buy books from the Connect Store, it'll deduct them from your credit, until it's all gone, then it'll want a credit card for further purchases.
