You have to save the world in a compressed file. Then go to mediafire.com (you have to join if you haven't already, its free) and upload your file. When it is done, you should see a "share" button, click it and copy the link to your world file. Come back here to PMC, go to your submission and click "update project" at the very bottom of the page there will be a world download URL. Right click and paste the link you copied earlier from mediafire, then click save and you should be all set. Hope this helps.