Meteorites in Rage 2 are random events (with some scripted exceptions) that can happen pretty much any time you play the game. You will clearly see the meteor fall down from the sky, and after this you can go and loot it any time.
To find the meteor that fell down, look for a blue circle on the map. The even is called Feneleatries on your map, so make sure to head to this location. When you get there, use a gun or another weapon to break it open and enjoy the loot that is inside it!