Note: You should always refer to a mod creator’s official installation description before installing a mod. Likewise it’s recommended that you create a backup of your Minecraft directory before attempting any modifications to avoid loss of data and ensure you have a stable version to fall back on.
To access the directory needed to install Minecraft modifications you will need to locate your root Minecraft directory.
Begin the RUN application (Windows Key + R) and type '%appdata%'. Here you will find the .Minecraft folder, open it.
The Minecraft folder is found in the following directory - /home/Your User Name/.minecraft
The Minecraft folder is found in the following directory: ~/Library/Application Support/minecraft
Once you have located the directory enter the .bin folder. Inside the .bin folder, use a directory browser (winZIP, Win RAR etc) to open the Minecraft.jar file, this is where modifications must be installed but the exact installation varies depending on the file so please refer to the author’s description from this point.