Get in the zeppelin. Go away from Berlin, then turn around and gently glide towards Berlin. If you go slow enough, the zeppelin will momentarily get low to the ground and glide straight through the big building in Berlin. You will come out by some big rocks, and you will here a splashing sound (as if the zeppelin hit the water.)
Search the Amphibious Vehicle cheat on this site. Once the tank is in Germany, drive it up the road to Berlin (the fortified place with the zeppelin.) Bring the tank into the doorway, stopping you from getting in and out. You can jump over the tank, but cannot walk to get in. As Indy, whip a character you have bought that is currently outside of Berlin. Jump over the tank and go into Berlin. The person will still be whipped, but they will be stuck outside of Berlin (they cannot jump while whipped.) Go to the statue in the middle of Berlin and walk around it several times, wrapping the whip around the statue. This will cause you to shoot up in the air very high. When you fall, it will look like you are going to hit the big rocks by Berlin, but you should land on the wall by the zeppelin.
Go to Raiders Of The Lost Ark. Go to the desert and get in the tank. Roll up the ramp and drop into the river, being careful to land right side up on the little beach. Then you can float around in the river, creating an amphibious vehicle. You can cross the river and go into Germany and Italy.