upload

I'm trying to upload a map to the website here and I get the error "You must upload a file to create a version or download." However, I already have a file uploaded, so I can't post the map. I'm not sure what I'm doing wrong here, I have every other field filled out. I've tried uploading a...

I know it's a stupid question, but can someone tell me how to upload it there, or if there are any tutorials online? (I already checked this, but they are either nonexistent or extremely vague.