Once you have received a child's transfer code, you should go to the "Families" tab. At the top right, you will find a green button named "Add new family".
After pressing you must click on the "Yes, I have the transfer code" button.
Here the code will be entered and the group where the child will be added will be selected.
After pressing the "Add in Kindergarten" button, the child will be active in the school/kindergarten.