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Step 1 Download Risugami's Modloader if you haven't already . link: http://www.minecraftforum.net/topic/75440-v166-risugamis-mods-updates/
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Step 2 Open up the Minecraft folder following one of the following 3 steps, depending on your OS (operating system) . In Windows XP: Click on the Start button, then click on Run. Type %appdata% and click Ok. Minecraft should be the first folder. Open it. In Windows Vista/7: Click on the Start(Windows) button and type %appdata% in the search bar. In Linux Ubuntu(should be the same on other Linux Distributions):Open up your Home folder. Look for the Minecraft folder.(note if you cannot see this folder, enable "Show Hidden Files". When you find it open Minecraft.
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Last 5 Steps Step 3 Open Bin folder . Step 4 Open the minecraft .jar file with 7zip or similar. Step 5 Drag the .class files from the Modloader folder into the .jar. Step 6 Delete Meta-inf, close the .jar and run Minecraft.exe. Step 7 Log in like normal . ModLoader is installed