Yes, the URL has to end with a /
Also make sure you put all your files in the Public folder, otherwise the redirect wont work.
to get the URL to use you'll want to go into the public folder on the website, right-click a file, click "copy public link" or whatever it says and you'll get a URL like this:
http://dl.dropbox.com/u/1252584/Arena.usx.uz2
take that url and remove the map name part, leaving the last slash.
If any of the maps you use need textures or other files other than the map, you need to compress those files also so they have the uz2 extention, then put those files in the public folder too.