I am trying to spawn a p3at in gazebo. [ubuntu 14.04/ros-indigo]

my launch file is;

<launch>

<!--##########################################################################-->
<!--Launching gazebo world -->
<arg name="world"  default="my_world"/>
<include file="$(find gazebo_ros)/launch/empty_world.launch">
<arg name="use_sim_time" value="true"/>
<arg name="debug" value="false"/>
<arg name="gui" value="true" />
<arg name="world_name" value="$(find scca)/worlds/$(arg world).world"/>
</include>
<!--##########################################################################-->
<!-- Convert an xacro and put on parameter server -->
<param name="robot_description" command="$(find xacro)/xacro.py $(find p3at_description)/urdf/p3at.xacro" />

<!-- Spawn a robot into Gazebo -->
<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-param robot_description -urdf -model p3at" />    
<!--##########################################################################-->

<!--##########################################################################-->

</launch>

I am following tutorial given here

But unable to spawn.

it throws an error,

 Package[p2os_urdf] does not have a path, 

i am unable to understand the error.