Acquainting with Chunk Definition & Arc Files
When it comes to modding in new content for these games, there are two files that you must understand how and why they are used. Neglecting these two files, will in a lot of cases make your mods not work.
This is a cache file, which contains pre-generated data for the game. This makes the initial load time of the game faster. However, keeping this file in your game folder will make any mod you have installed not load/work.
The best course of action is to either remove the file completely from your game directory, or open it in a hex editor and delete everything inside. This is personal preference, but you will need to choose one of two methods.
This will make your game load slower depending on your system specs.
This process is not the same for console game modding.
The Chunk0.def (abbreviated as 'def file' through-out this wiki) is a definition list. It contains a list of all files inside your game directory. When installing a new mod to the game, you will need to generate a new def file for the game to read your new files.
You do not need to regenerate a def file if you are only replacing an existing vanilla file.
This may sound daunting, however there are tools that exist to make this an easier one click process. The most known, widely accepted and used is Custom Character Tools (CCT) found here.