If your know the URL of the Front Desk Web App, open it. Enter your username and password, then click on the button "Log In" at the bottom of the page.
Please bookmark the app's URL so you can find it easily next time you want to log in: http://frontdesk.zabcab.com. You may also want to check the "Remember me" option in the login page for fast login in the future.
For your convenience, the app's URL is provided in the welcome email you received when you created your Front Desk Web account, under the on the button "Start Getting Cabs" at the bottom of the email.