You obviously are being a smartass, or you're just not very perceptive. He's asking about the mod SinglePlayerCommands. Not the Cheats. If he wanted the build-in commands, he'd say "How do I use single player commands" not "How do I download single player commands".
You get commands built in since 1.3.1. When making a world, make the cheats setting ON. If you want to upgrade your world that you made without cheats, open to LAN and allow cheats. You'll have to do the LAN thing whenever you re-join the world.