It's been a while since I've done the secondary, but I think the additional information is for anything you want to explain on your transcripts (ex: bad grades, misdemeanors, etc). Can you please post the prompt so I can make sure I'm telling you the right thing?
The diversity essay is basically how you can "contribute to the diversity" of Mizzou's class. This part was easier for me because I'm an ethnic minority, but a lot of my classmates wrote about various diverse experiences, for example traveling to other countries, teaching, surfing, doing research, living in a co-op, etc. It turns out that our class composition is for the most part, similar to the demographics of the state itself (very white and middle class), so obviously you don't have to be an ethnic minority to be accepted
. Basically it's like what different perspectives and ideas can you bring and share with a PBL group.