roslib.load_manifest error - Python

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I'm trying to convert the points in PointCloud2 Messages to a numpy array, now, my main problem is whenever I try to use this line:

import roslib; roslib.load_manifest('python_msg_conversions')

I'm importing the roslib etc.. I get the following error:

$ python talker.py 

Traceback (most recent call last):
  File "talker.py", line 1, in <module>
    import roslib; roslib.load_manifest('python_msg_conversions')
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslib/launcher.py", line 62, in load_manifest
    sys.path = _generate_python_path(package_name, _rospack) + sys.path
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslib/launcher.py", line 93, in _generate_python_path
    m = rospack.get_manifest(pkg)
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 159, in get_manifest
    return self._load_manifest(name)
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 198, in _load_manifest
    retval = self._manifests[name] = parse_manifest_file(self.get_path(name), self._manifest_name)
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 190, in get_path
    raise ResourceNotFound(name, ros_paths=self._ros_paths)
rospkg.common.ResourceNotFound: python_msg_conversions
ROS path [0]=/opt/ros/hydro/share/ros
ROS path [1]=/home/O/src
ROS path [2]=/home/catkin_ws/src
ROS path [3]=/opt/ros/hydro/share
ROS path [4]=/opt/ros/hydro/stacks

I have no idea as to why I'm getting this error either.

I hope someone can help me