Here are steps to where to enter your Virginia 529 contribution amounts: 

Even though these are contribution payments, the entry section is in the Income section of the Virginia tax return interview.  On the page with the heading, "Here's the income that Virginia handles differently", scroll down to Education and select to Start the Virginia College Savings Plan Tuition (529 Plan). This will bring you to the entry section for your contribution amounts.  See the sample screenshot below.