Before you can use FieldLab Desktop to administer Engineering Units, you must first Add the FieldLab to FieldLab Desktop. Full instructions for doing so are here: Add a FieldLab to FieldLab Desktop
Select the FieldLab from the FieldLabs screen.
Go to the Settings Tab and select the Change button in the Engineering Units box.
Select the Engineering Unit you'd like to Add or Remove from the gauge.
In the pictured example, the engineering unit "bar" is selected to be added to the FieldLab.