Check the PM I sent earlier this morning, this is the top level URL:
http://www.subsim.com/mods1/gdc3/
When you add a file, just use this for the link in the Downloads Add file section:
http://www.subsim.com/mods1/gdc3/example_file.zip
That is the URL that you use to set up your download.
Feel free to PM me if you have any questions, I can help you work through them.
Neal