Great! I suppose you want to know what technology we use?
OK, that wasn't helpful? Get a copy of the source using git, see http://sourceforge.net/scm/?type=git&group_id=394854 for some information about git. Also http://git-scm.com. Some information about the repository is listed on ../source-repository.html and on the sourceforge page listed above.
Once you've got a copy of the sourcecode, get a copy of the Eclipse IDE from http://eclipse.org either get the IDE build for Java developers or the EE build. Personally I use the EE build as I prefer those build-ins. You may also want to install the eGit plugin http://eclipse.org/egit/.
If you want to install and build the project you will need to get a copy of maven 2 from http://maven.apache.org and you'll need to get a copy of NSIS from http://nsis.sourceforge.net and build that for yourself. You will need to put the NSIS executable on your path for maven to find it. If I've done my work correctly once you've done that, if you run mvn install in the base project directory you should be able to build a copy of the program and the installer quite easily. Get back to me if you can't.
In Eclipse, I would add the editor and classpath-alterer sub-projects rather than add the base directory as the eclipse project. This makes life easier when working on the project.