After two months driving my 2018 I3, I finally stumbled onto how to adjust the height of the driver seat. While standing next to the car (not sitting in the seat), pull the front lever up, and the seat will raise to its maximum height. I had tried to raise it while sitting in it, following the instructions to try to lift my weight off the seat, with no success. Once the seat is raised, you should be able to lower it as desired while sitting in it. I will probably leave mine at maximum height, since I'm not particularly tall.
2 0 1 8 B M W i 3 9 4 A h B E V - Los Angeles