Put a link to the installer download at the top of every page of the forum, especially the index. Make it so the banner in the Ad box is a link to the installer page of the wiki. Bottom line: Someone who has never heard of Aurora before should be able to do a search for "Free 4x games", go to any of the top-ten results, then click no more than two links (large, obvious links) to start downloading. Perhaps an even better rule is this: you have ten seconds to get someone to download your game, or they are gone for good.

I never would have found the installer myself if I hadn't been given a direct link to the wiki page from the Dwarf Fortress forum.

Also, consider breaking the main index into several sections. Make each forum have a separate index page to cut down on "menu panic".