I don't know what your background is, but to give a broad outline, you have to apply to the java source downloaded from github then rebuild the Arduino project. http://code.google.com/p/arduino/wiki/GitandGithubGuideforArduinoDevelopment
To apply the patch, use the 'patch' command from the command line on unix, or cygwin. Or you can use the command 'git apply'
Typical usage would be:
$ cd <root of the source tree>
$ patch -p1 open-recent-file.patch
Then use 'ant' to re-build the project.
You could also pull the branch I posted from github just compile it if you just want to see what it looks like.