Please make sure that you have the latest version of the Passenger App (version 17.3 for both iOS and Android.) The button is not available for older versions.
You will find the button at the bottom of the screen next to the “cancel booking” button when a ride has been booked but not yet started.
For more information about “Track my ride” please check our user manual here: http://www.manula.com/manuals/taxicaller-nordic-ab/passenger-app/1/en/topic/track-my-ride