It should just be a straight forward address to the file, usually zipped up and hosted somewhere.

Example;
http://backroom.renderosity.com/~bryce/newsite/challenge/shared_files/irrdesmats_b5.zip

Or, you can insert a url which would take you to the page that holds the item to download. Like this;
http://backroom.renderosity.com/~bryce/newsite/challenge/shared%20pages/textures_mats_02.htm

It's usually best to copy/paste the url into the field required, as it just takes one little mistake for the url not to work at all.

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