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 default screen, from here you can open the side menu and select Login menu item. On the login page, enter your Username and Password to login. If you have forgotten your login details please use the 'Forgotten' button provided on screen, or see the 'Reset 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 Pay & Stay map to do so without logging in. Please see 'Paying for Parking without an Account on iOS' for more information.