Do what Malivil told you.
[*]Download minecraftforge-universal-1.4.6-6.5.0.489.zip to the same folder that minecraft-server.jar is in.
[*]Rename minecraftforge-universal-1.4.6-6.5.0.489.zip to minecraftforge-universal-1.4.6-6.5.0.489.jar.
[*]Launch minecraftforge-universal-1.4.6-6.5.0.489.jar as if it were minecraft-server.jar.