In order to log in offline, you will need to Update Member Records at least once when you have access to the internet. This is what encrypts the data to your phone to allow for offline viewing.
When you have access to the Internet, log in and click on "Go to the Update Records Page."
Then, click on Update ALL NOW or select the organization or group you wish to update.
You can always return to this page by clicking on Menu in the far top right.