Per the IRS Instructions for Form W-2 (Instructions for Employee, Box 10):
Box 10. This amount is the total dependent care benefits that your employer paid to you or incurred on your behalf (including amounts from a section 125 (cafeteria) plan). Any amount over $5,000 is also included in box 1. You must complete Form 2441, Child and Dependent Care Expenses, to compute any taxable and nontaxable amounts.
To enter child and dependent care expenses, click the Federaltab, then click Child and Dependent Care.