To add a new vehicle, go to Account > My Vehicles:
Here you will see any current vehicles. A maximum of 3 vehicles can be added to each account (only 1 can be used in a car park at any given time).
To add a new vehicle click "Add New Vehicle":
Now you can add the vehicle registration details and select if this will be your default vehicle. Setting the vehicle to default will simply mean it is shown first when you are required to select a vehicle for things such as bookings.
Once you have entered the details, click Add Vehicle:
To remove a vehicle, click on the vehicle and then click on the Delete / Trash Can icon in the top right.