Things You'll Need
Instructions
Open the "Users/YourName/AppData/Roaming" file directory on your main hard drive.
Open the ".minecraft" folder, and then open the "bin" folder.
Open the "minecraft.jar" file with an unzipping program such as 7zip, WinRAR or Winzip.
Delete the "META-INF" file in the "minecraft.jar" file.
Double-click the More Worlds Mod to open it with your unzipping program. More Worlds Mod is available at "Minecraft" sites such as Fatal Effect, "Minecraft" Forums and Mods for "Minecraft."
Copy the two ".class" files from the More Worlds Mod into the "minecraft.jar" file. Close "minecraft.jar" and the More Worlds Mod folders.
Load "Minecraft" and select "Single Player." You will now have more save slots.