The entrance to the gardens are on, and visible from, R street, directly across from 31st NW. The door to the museum is a bit trickier to find. If you are on R street head towards 32nd Street NW. Turn north on 32nd and about halfway up the street you will see large double doors on your right. Those doors will take you to the museum.
I am a former employee from Dumbarton Oaks and I am confirming that the directions provided by Troy Stopera above, is accurate and correct. Just follow those directions and you will find your way to both the gardens and the museum. Enjoy the visit! It is one gorgeous place!
You'll see it marked from the main Street
Thanks! Your answer is awaiting moderation.