Running client in debug mode
From Mod Coder Pack
- Minecraft installed in %appdata%\.minecraft
- The Linux minecraft.jar from the Minecraft download page 
- Ability to run commands on the computer. (On Windows, typically this needs admin rights.)
- Make sure Minecraft is installed.
- If you haven't done so already, download the Minecraft.jar from the Minecraft.net download page. (linked above)
- Save that minecraft.jar in a folder of it's own.
- Hit Start->Run or Start->Search on Win7 and type cmd and hit enter.
- In the command prompt, type cd <path where you saved the linux jar> replacing <...> with the actual path.
- If you saved your file on a different drive than C: then type <drive>: replacing <...> with the drive letter you need.
- Now make sure you have your PATH system variable set up to Java. (go HERE if you don't know how)
- Type Java -jar minecraft.jar and hit enter!
- Minecraft installed in ~/.minecraft
- Some kind of JRE installed and working
wget https://s3.amazonaws.com/MinecraftDownload/launcher/minecraft.jar java -jar minecraft.jar
Seeing the results
Your game should now load up and in the background your console window will still be up, showing you lot's of information on what's going on in the background. This includes any System.out.prinln(); calls you have in your modified code.
If you have ModLoader installed in your game you will see a line like "Mod Loaded: <ModName>" for every mod you have installed in the client that uses ModLoader.