You can use any IDE for Nachos. Eclipse is a good tool, but it requires some setup. Here are some tips that have been collected; please send any tips you have.
Nachos has a directory/package structure that Eclipse doesn't like very much, so you have to be careful when creating the project so that Eclipse can find each package. (Otherwise it might see "threads" instead of "nachos.threads".) You should create the project in the parent directory of your Nachos source tree, i.e. the directory containing the directory "nachos".
One way to do this is:
The Subversion plugin for Eclipse is Subclipse.
To install from Eclipse: Help → Software Updates → Find and Install → Search for new features and Install
To add a new repository: Window → Open Perspective → Other → SVN Repository Browser. Right click on blank spot in browser and choose New → Repository Location.
To export your current project in your PC onto the repository, just switch perspectives back to "Java" perspective (clock on Java near top right corner of eclipse window), right click on your nachos project and choose: Team → Share Project.
To change the background color go to preferences → window → java editor → background color.
The main class to run is nachos.machine.Machine. If you follow the example above, this will be set in Eclipse.