Loading Zone #1 is reached soon after the first incursion into the maintenance tunnels. The orange symbol shows the location where Cortana first gives you a navpoint - this is where you enter the tunnel. Take your first two rights, then a left and a right... and you're there. It's safest to pass through the zone twice (once forwards, again backwards), to be sure it 'sticks'.
Watch the movie for Zone #1
Loading Zone #2 is in the second set of tunnels you reach - just past the cryo-chamber observation deck.
Watch the movie for Zone #2