I have noticed now that CraftBukkit has been taken down by a DMCA that more and more people are trying to use Bukkit instead of CraftBukkit. When they do this they receive the error: "no main manifest attribute, in craftbukkit.jar" or something similar. If you are receiving this error it is for a simple reason. You are trying to use Bukkit instead of CraftBukkit. You may ask, "what is the difference?". Well Bukkit is the API that plugins use. CraftBukkit is the 'thing' that runs the server. So when you try to use Bukkit instead of CraftBukkit it will not work. Since you cannot currently download CraftBukkit legally, you can no longer make a CraftBukkit server unless you download CraftBukkit before it was taken down. I hope this helps, please let me know if I have made any errors.