How do I add the 401K or 401k Roth deductions on the W-2?

Go to Payroll, Tools, Modify Forms, W-2 Forms.
Select the correct W-2 Form.
On the Amounts tab, click in one of the Box number content fields.
For example, use Box 12 or 14.
Click Deducts.
Select the 401K deduction ID.
Enter the appropriate W-2 code for the box content description.
Tip: For a full list of box 12 codes, see the Payroll Help Topic "About box 12 on W-2 forms."
Note: Description box is located immediately to the right of box 12, it can be dificult to see with some displays.
Click Save.