Sorry, rosstack looks for stack.xml, not manifest.xml. Try using "rospack find pcl". rospack seems to use folder names, so you should symlink /usr/share/pcl -> /usr/share/pcl-1.6. Looking back at your original error message, it looks like the command "rospack deps-manifests costmap_2d" is what is failing. So the "rospack" command should be the one you're using to find PCL. I just tried creating the symlink, installing manifest.xml, and running the rospack command to find PCL and find the costmap deps, and it seems to have worked.
Failed to invoke /opt/ros/fuerte/bin/rospack deps-manifests social_robot[rospack] Error: package/stack social_robot depends on non-existent package opencv2