If you are creating new packages or want to edit existing ones, you need to put the corresponding source code in the sandbox directory or another directory that has been added to your overlay with rosws set. For instance, if you want to edit the package erratic_robot, add a copy to your overlay. The copy will "shadow" the package installed in /opt/ros/fuerte, i.e. when using roslaunch/rosrun, the system will use that one in your overlay. You can add the source code of erratic_robot as follows:
rosws set --svn erratic_robot https://ua-ros-pkg.googlecode.com/svn/stacks/erratic_robot/trunk

Then you need to build it: rosmake erratic_robot.