The Sylvan Theater is located at the lower right in the above image, among the trees on the monument grounds. The road running from the top of the image to the bottom, on the right of the Monument is 15th Street. No parking along there. If you look in the very bottom right corner of the image you will see a road disappearing off the image... that is Independence Ave. You may find parking along there. Otherwise you may find parking along Constitution Ave, which runs parallel to and north of Independence Ave. You may also find parking on 15th Street north of Constitution Ave intersection.
If you drive in and park, you may have a several block (possibly more) walk. If you take the Metro in to the Smithsonian stop, you will walk approx. 2-3 blocks.
I hope this is helpful.