I am new to ROS and was trying the pr2 Simulator tutorials. In the very first step of rosmake pr2gazebo i get a responzse like this- rosmake pr2gazebo [ rosmake ] rosmake starting...
[ rosmake ] Packages requested are: ['pr2gazebo']
[ rosmake ] Logging to directory /home/sam/.ros/rosmake/rosmakeoutput-20140408-074409 [ rosmake ] Expanded args ['pr2_gazebo'] to: []
[ rosmake ] WARNING: The following args could not be parsed as stacks or packages: ['pr2_gazebo'] [ rosmake ] ERROR: No arguments could be parsed into valid package or stack names.

I dont knwo how to fix this error. Please help.