It was on the airplane, not the shuttle, see this NASA article:
A crew escape tunnel was installed aboard NASA 747 aircraft number 905
(NASA 905) during the aircraft's modification process for the Shuttle
program. The tunnel extended down three decks, from the flight deck to
the bottom left side of the fuselage. In a catastrophic emergency, the
parachute-clad pilots and flight engineer would activate explosives
that would blow a hole through the fuselage for bail-out. Next, they
would slide down and out the hole, escaping into the air below the
airplane. Activation of the tunnel hole explosives also activated
pyrotechnic devices designed to blow out 10 windows above each wing in
order to equalize the on-board air pressure allowing the crew to slide
down the tunnel. However… this escape system might have led into the
inboard engine inlet, obviously not a good thing! The escape tunnel
system was removed from the aircraft following completion of the Space
Shuttle Approach and Landing Tests (ALT) project.
The shuttle Enterprise was used for the atmospheric ALT, it had ejection seats for the crew. There was no way to get between the SCA and the shuttle.