I can help you if I knew what was going on exactly, but here's how I did it. Put the server jar (I would rename it to nukkit.jar if it isn't already) into a new folder on your desktop or someplace. Click on your desktop background so it says "Finder" in the top left on your screen. Then, click on the folder that has nukkit.jar in it (but don't open it, just a single click). Click on "Finder" at the top left of your screen, then "Services," then "New Terminal at Folder." Then type in "java -jar nukkit.jar" into Terminal. That should run the server.
