To open a match using the mobile app you have to access the main menu.
Then press on System Play!
Here you will see a few sections with information about the play! system. In this case you want to press on + Create New
Once inside you can select the sport, the date and the time you want to create the match. Once done, press on FIND
Then you'll see the list of courts available at that time for the specified sport.
Select one of the available courts and you'll see more options for the match, select the gender of the match if you like and an empty space on the match to join, pressing where the picture shows.
After that you'll have to select the payment method.
And once done, you'll see the confirmation screen: