map building

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I am new in ROS and self studying about this. Now I am working on map building using turtlebot simulation. I came across map building and navigation. When I tried (page:7):

$ roslaunch ros_liv_turtlebot office_turtlebot.launch
it throws the following error:

... logging to /home/afroza/.ros/log/0ea56abe-55ce-11e4-ae6d-0023aee70050/roslaunch-afroza-XPS-730X-4531.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

Traceback (most recent call last):
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/__init__.py", line 279, in main
    p.start()
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/parent.py", line 257, in start
    self._start_infrastructure()
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/parent.py", line 206, in _start_infrastructure
    self._load_config()
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/parent.py", line 121, in _load_config
    self.config = roslaunch.config.load_config_default(self.roslaunch_files, self.port, verbose=self.verbose)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/config.py", line 428, in load_config_default
    loader.load(f, config, verbose=verbose)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 698, in load
    self._load_launch(launch, ros_config, is_core=core, filename=filename, argv=argv, verbose=verbose)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 670, in _load_launch
    self._recurse_load(ros_config, launch.childNodes, self.root_context, None, is_core, verbose)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 605, in _recurse_load
    n = self._node_tag(tag, context.child(''), ros_config, default_machine, verbose=verbose)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 95, in call
    return f(*args, **kwds)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 351, in _node_tag
    self.opt_attrs(tag, context, ('machine', 'args', 'output', 'respawn', 'cwd', 'launch-prefix', 'required'))
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 178, in opt_attrs
    return [self.resolve_args(tag_value(tag,a), context) for a in attrs]
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 159, in resolve_args
    return substitution_args.resolve_args(args, context=context.resolve_dict, resolve_anon=self.resolve_anon)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/substitution_args.py", line 299, in resolve_args
    resolved = _resolve_args(resolved, context, resolve_anon, commands)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/substitution_args.py", line 312, in _resolve_args
    resolved = commands[command](resolved, a, args, context)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/substitution_args.py", line 137, in _find
    return _find_executable(resolve_without_path, a, [args[0], path], context)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/roslaunch/substitution_args.py", line 173, in _find_executable
    full_path = _get_executable_path(rp.get_path(args[0]), path)
  File "/usr/lib/pymodules/python2.7/rospkg/rospack.py", line 190, in get_path
    raise ResourceNotFound(name, ros_paths=self._ros_paths)
ResourceNotFound: gazebo_worlds
ROS path [0]=/opt/ros/hydro/share/ros
ROS path [1]=/home/afroza/catkin_ws/src
ROS path [2]=/opt/ros/hydro/share
ROS path [3]=/opt/ros/hydro/stacks

Can anybody tell me how can I fix this? your help is ... (more)