In this mission, you encounter a drunken Madd Dogg on the roof of the casino, onlookers are persuading him to jump, however you need to save him. Follow the game's instructions, to collect the pickup truck, and park it in the marker. Then manuever the truck as Madd Dogg stumbles and falls, and take him to the hospital.
When the view changes, do not steer at all, merely drive forward and backward, to match the position of Madd Dogg as he shuffles along the roof.
When he jumps, it LOOKS like he will not hit the truck, but if you have not steered, he will land on the boxes in the back of the pickup.
After that, drive steadily to the hospital. There is no timer, so you can take it easy (since bumps and knocks reduce Madd Dogg's health)
Make sure you haven't entered the "Ped Riot" cheat or completing this mission is impossible. Just don't save with the cheat on.