If you haven't already done so, download the MiPermit app for iOS devices from the Apple App Store.
After you launch the app you are greeted with the login screen, from here you can enter your Phone/Email/Username and PIN/Password to login. If you have forgotten your login details please use the 'Forgotten...' link provided on screen, or see the 'Reset PIN/Password' help article.
If you do not have a MiPermit account and wish to just pay for parking and have none of your details saved, you can use the 'Browse Locations...' link to proceed. Please see 'Paying for Parking without an Account on iOS' for more information.