You need an app that is compatible with passbook. In that app there will be an option to add your info into passbook. There are only a few so far from what I understand. The only one I have is the Walgreens app.
Ok, the key to this app is to not open the app directly. You must open an app that is compatible with passbook. For example, Starbucks.
When you open the Starbucks app, you can then update your information on your Starbucks account, so that gift cards, coupons, and preloaded $ are updated. Your credit card info is also an option, so that you can reload your loyalty card; you can also select specific locations.
For more info, go to www.passsource.com. You'll also get a list of current passbook friendly apps. Good luck!
See the other Passbook thread. One post has a link with explanation and a list of Apps that work with Passbook...Starbucks, Walgreens, etc. I got to create a fake airline boarding pass there which I could see in Passbook and eventually delete.
Looks like this site is for creating Passbook cards.
To use the Passbook App you must have to have the app and a card loaded for the vendor... Starbucks, Delta Airlines, etc. I don't have any cards so wasn't able to test for real.