If you're taking the Metro, it's right inside the doors the Metro shuttle drops you off at. That's how I found it
If not, there are signs everywhere saying where the Hall of Nations is. Once you're there, just go towards the doors to the driveway out front. Just inside the doors to the left (when facing doors) is the opera house stagedoor. It's 2 sets of double doors and they're labled "opera house stagedoor" so it shouldn't be too difficult to find.
WAT, I'm seeing Mame this summer as well.
Like a firework unexploded
Wanting life but never
knowing how