It does not work for me either, though it may be just a temporary glitch.
Spaceempires.net hosts most, if not all mods. Both Proportions 2.5.XX and the 3.0 Version are available on
this page. If you only play against the AI, you will want to stick with the 2.XX.XX Versions though, as the 3.XX Versions do not have a fully compatible AI yet.