When you have alerted the horde, head half-way up the ramp. Make sure to turn around and walk across the gate on the right.

Be sure to eliminate some infected climbing on the gate if you're in a Mutation/Campaign.

Once walked across, you can go under the part of the coaster and get to the ladder.

Beware: Just make sure to watch the coaster cars and wait for them to pass, otherwise, you are dead!