Instructions
Download the "Darkfirebird's Class and Pet Mod" from MegaUpload (see Resources). The mod is available for free and will be downloaded in an .exe format.
Double-click the .exe file. A standard installation window will appear.
Agree to the mod's Terms of Use. It will then ask you for the directory to install the file.
Locate the "Mod" folder in your "Torchlight" directory. This directory can be located in a variety of places, depending on where you installed the game. By default, the game's file path is "Program Files/Steam/User Name/Application Data/Runic Games/Torchlight/Mods." The name of your Steam Account will replace the "User Name" folder.
Click "Install" to install the mod. You will now have 18 classes to choose from in Torchlight.