Thanks, ahendrix.
Now, I can copied the source from the repository and put it into ROS_PACKAGE_PATH.
But I am still stuck.
When I typed 'rosmake pr2_navigation', I got the following
[ rosmake ] rosmake starting...
[ rosmake ] Packages requested are: ['pr2_navigation']
[ rosmake ] Logging to directory /home/jiwung/.ros/rosmake/rosmake_output-20140713-161122
[ rosmake ] Expanded args ['pr2_navigation'] to:
[]
[ rosmake ] Results:
[ rosmake ] Built 0 packages with 0 failures.
[ rosmake ] Summary output to directory
[ rosmake ] /home/jiwung/.ros/rosmake/rosmake_output-20140713-161122
jiwung@jiwung-HP-2000-Notebook-PC:/opt/ros/fuerte/stacks$ rosmake pr2_navigation_perception
[ rosmake ] rosmake starting...
[ rosmake ] Packages requested are: ['pr2_navigation_perception']
[ rosmake ] Logging to directory /home/jiwung/.ros/rosmake/rosmake_output-20140713-161135
[ rosmake ] Expanded args ['pr2_navigation_perception'] to:
As expected from the above, 'rospack find pr2_navigation' results in
[rospack] Error: stack/package pr2_navigation not found