Installing Custom Maps
Most .zip and .rar files on this site are meant to be installed to your svencoop_addon folder, which is usually C:\Program Files\Steam\steamapps\common\Sven Co-op\svencoop_addon. Everything will automatically go where it's supposed to.
Simply start Sven Co-op, go to Start New Game, and pick the map you want to play. Or go to Join Game and pick a server.