You see the current layout of one label in "Edit Mode" on the left.
Note: There might be more than one label on a piece of paper, but the whole edit area on the left is just one (enlarged) label.
Select "Run"-"Run" in the menu to test the current layout and to display the resulting PDF.
Pressing "Label" brings you back into "Edit Mode"
Select "Insert" from menu to add a new field to your label.
For some field type the property dialog will open. Adjust the field attributes and press "Ok".
Drag and drop the field into the desired position and size.