Congrats on making it to the waitlist -- while it's not anyone's first choice, it still means that you have a shot!
We've got some advice in the Lab on how to "navigate" the waitlist. It's a little hidden at the moment, but you'll find it in the "To Do List" module (at the very bottom under "The Interview and Beyond").
Know that in most cases, being on the waitlist isn't a matter of them thinking that you have weaknesses that you need to address - it's almost always the case of "we like this person, but let's wait to see who we get in upcoming rounds who might be similar but just a smidge stronger" - schools use them as a tool to manage class make-up and yield/enrollment.
Bottom line: follow their instructions. If they explicitly say that they don't want/need any additional information, then honor that.
Hope that helps and good luck!