To upload a project, you'll either need to compress your world so that you can upload it onto a file sharing site (i.e. Mediafire) and place that link in the "World Download URL" at the bottom of the page,
or
you can use MCEdit or Single Player Commands to make a schematic. Schematics can be uploaded to PMC with just the "UPLOAD FILE" function next to the "MCEdit Schematic" text at the bottom of the page again.
You can upload either or both, it doesn't really matter as long as you provide some download.
To compress your world with a mac, just right click and click Compress. Your save world folder should be there still, but a new file should appear with the same name but different look with a .zip in front of it. If you are not sure where your world saves are, it's actually just in the Minecraft folder. If you're not sure where your minecraft folder is, use the spotlight function to help you.