Don't stress the essay. I am awful at spelling and constantly reply on spell check (i even go as far as copy the post into word, spell check it and then paste it back for posting) and I managed to get in. The topic are usually dental related and are very very broad so you have no touble forming an opinion and coming up with examples or solutions.
Good luck. Arrive early (parking can be a bit**, I suggest parking in the CHS parking lot off of Tiverton Dr. and Le Conte Ave. Just pull up to the parking kiosk and ask for the patient/visitor parking, it's $8 for the day so you will not have to worry about meeters. Also bring a pencil with a good erasor so that you can neatly fix any mistakes on the essay. Other then that, relax and you will be fine.
P.S. You will meet up for lunch with a student and that is going to be your student interview so don't start talking about keg-stands.