Which exact version do you try to download?
How do you check if the plugin is installed?
I tried to download customcrafting-1.6.4.0.jar, I know I have it installed by following the route
Files > Plugins, and I can see a file named customcrafting-1.6.4.0.jar. But when I load up a world the plugin does not function (no commands linked to the mod work)