cannot add erratic urdf in gazebo

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Hello everyone,

I am using gazebo 1.9 with ROS Groovy. I want to add an erratic robot in the willowgarage world. In order to do so I have the follwing launch file:

<launch>

  <!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="worlds/willowgarage.world"/> <!-- Note: the world_name is with respect to GAZEBO_RESOURCE_PATH environmental variable -->
    <arg name="paused" value="false"/>
    <arg name="use_sim_time" value="true"/>
    <arg name="gui" value="true"/>
    <arg name="headless" value="false"/>
    <arg name="debug" value="false"/>
  </include>

<param name="robot_description" command="$(find xacro)/xacro.py $(find erratic_description)/urdf/erratic.urdf.xacro" />

<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-param robot_description -urdf -model erratic" />

</launch>

This example file worked perfectly with pr2, but when I launch it I get the following error:

[rospack] Error: stack/package smart_arm_description not found
[librospack]: error while executing command
[FATAL] [1374747130.949047781]: Package[smart_arm_description] does not have a path
[spawn_urdf-4] process has finished cleanly

I cannot find this smart_arm_description package, can someone help me?

Thank you in advance