When you're logged into the app, tap on the menu icon in the top left of the screen. Then go to My account and Buy a WiFi pass. Here you'll see all passes that are available to buy in your location, ordered from the shortest length of time to the longest. Once you've found the pass you want to buy, tap on it, select your preferred payment method and follow the steps. You can currently buy passes using a credit or debit card, or PayPal.