The code DD for Box 12 on your W-2 is in the drop-down list after code D (the seventh item down the list). If you have other lines in Box 12 to add, be sure to click the blue + Add Another Row link to be able to enter everything from your W-2.
As user ScottT stated, the information reported in Box 12 for Code DD should have no effect on your return if entered with the correct code. If you are seeing something different, please provide more details.
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