Where do I enter my 529 contributions if I file in Washington, DC?

You will see an option for that in your DC tax program. It will not appear in the federal program, as your 529 plan contribution is only deductible on your state tax return.

This unfortunately does not appear to be the case for the DC tax return (at least not that I can find).

Click on the State Section of your return.

After you click through the pages about residency it has a page titled Items Taxed By DC.

The next page is Deductible Items in DC.  Scroll down to DC college savings plan.

This is where you enter your information about 529 contributions.