Click on the Create... button in the Units section of the Edit String dialog. The Create Unit dialog appears; enter the information and press OK to create a new unit under the string. Note that you can add units before they are actually physically available; indicated whether the unit is physically installed by clicking in the check box.
You can assign I/O devices to the unit by clicking on the Units/Devices... button to open the Units/Devices dialog. See Assigning devices to units in the string.