Ignore the plank, that isn't really the way to go.
After talking to the projectionist upstairs check his room for the power switch to the organ. Turn it on and head back to the seats you climbed to earlier. You'll see some guards, take them out using the crossbow.
Now, take the exit door and head right to the stairs- go down until you see the organ. Turn it on and climb on- this will take you straight onto the stage.