Ok, here is what you do. First, go into your Dropbox home. Then find the project you wish to make available to the public (in this case "The Creeper's Crusade"). Look to the right of the screen and find the picture of the chain links. This is the "share link" button. Click that and it should take you to a screen with a picture of the zip file. There in your address bar should be the URL you are looking for. Copy that and put it as your world download link. Well, there you have it. I hope this helps.