Note: The library is currently being refactored - there will be many breaking changes to the API! MCModify is Java library designed for easily modifying extisting Minecraft files. There are already other libraries like this for Java, Python, C#, C, C++, etc., but if you like the library interface, feel free to use it.
The repository is a Maven project and if you know how to use Maven you can easily build the project. If you don't know how to use Maven, just open the project in NetBeans (which has native support for Maven projects) and build it as you would build any NetBeans project.
There is also a version of the library in early development for C++14/C++1z, available from the cpp branch.