The NRG-500 Challenge is located at the Easter Basin docks in San Fierro. It is behind the crates near the very large and empty bowl. It is available to you as soon as you get to San Fierro, so you can do it right away.
This challenge is like the BMX one in Los Santos, only you get to control a very fast motorized bike. As was the case before, you start out with 10 seconds and gain an additional 10 seconds for each corona you get. Unlike the BMX where you could jump, you have to ride up the walls of the bowl to the rest of the coronas.
Simply gather all of the coronas on the dock and in the bowl, then line yourself up with the other ones before attempting to jump for them. Line up and speed up the slanted wall to grab them. If you fall off the bike, you have 25 seconds to return to it before you fail.