...the North Carolina Collection at the University of North Carolina at Chapel Hill to plan our first ever edit-a-thon. Our theme is African Americans in North Carolina,
...where is the best place on Wikipedia for me to promote this event?
There is no one place that serves as a central promotion spot. As with social media, you have to get into various places to promote your event.
- Presumably the people in the local Wikimedia chapter are apprised of the event and will come.
- Make sure the library and UNC will promote the event (widely) - don't forget the campus newspaper
- Social media: Twitter, Facebook, and the usual suspects
- There must be local venues/outlets (newspapers? websites? community centers?) that cater to the African American community in Chapel Hill and environs -- use them.
- I note the date is April 16 - might you want a tie-in to Lincoln?
In short, as with social media, there is no "one size fits all" and you have to reach out in many ways (often) to promote your event.
Good luck!