Gazebo: Invalid <param> tag: Cannot load command parameter [robot_description]:

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Hi.

I'm using Ubuntu 14.04 ROS Indigo Gazebo2

I am quite new to ROS and Gazebo and I'm following this Tutorial link:here.

I did everything like in the tutorial, nut when I want to use roslaunch to launch my launchfile I get the following error:

 Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/indigo/share/xacro/xacro.py 
 /home/myuser/catkin_ws/src/mybot_description/urdf/mybot.xacro] returned with code [1]. 

 Param xml is <param command="$(find xacro)/xacro.py $(find mybot_description)/urdf/mybot.xacro"   name="robot_description"/>
 The traceback for the exception was written to the log file

When I enter

rosrun xacro xacro.py /home/myuser/catkin_ws/src/mybot_description/urdf/mybot.xacro

I get

Traceback (most recent call last):
  File "/opt/ros/indigo/share/xacro/xacro.py", line 62, in <module>
    xacro.main()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 687, in main
    process_includes(doc, os.path.dirname(args[0]))
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 269, in process_includes
    % (filename, str(e)))
xacro.XacroException: included file "/home/myuser/catkin_ws/src/mybot_description/urdf/macros.xacro" generated an error during XML parsing: unbound prefix: line 2, column 0

My macro file looks like this

<xacro:macro name="box_inertia" params="m x y z">
  <inertia  ixx="${m*(y*y+z*z)/12}" ixy = "0" ixz = "0"
    iyy="${m*(x*x+z*z)/12}" iyz = "0"
    izz="${m*(x*x+z*z)/12}"
  />
 </xacro:macro>

And my mybot.xacro looks like this:

<?xml version="1.0"?>

<robot name="mybot" xmlns:xacro="http://www.ros.org/wiki/xacro">
    <!-- Put here the robot description -->

<xacro:property name="PI" value="3.1415926535897931"/>

<xacro:property name="chassisHeight" value="0.1"/>
<xacro:property name="chassisLength" value="0.4"/>
<xacro:property name="chassisWidth" value="0.2"/>
<xacro:property name="chassisMass" value="50"/>

<xacro:property name="casterRadius" value="0.05"/>
<xacro:property name="casterMass" value="5"/>

<xacro:property name="wheelWidth" value="0.05"/>
<xacro:property name="wheelRadius" value="0.1"/>
<xacro:property name="wheelPos" value="0.2"/>
<xacro:property name="wheelMass" value="5"/>

<xacro:property name="cameraSize" value="0.05"/>
<xacro:property name="cameraMass" value="0.1"/>                             


 <xacro:include filename="$(find mybot_description)/urdf/mybot.gazebo" />
 <xacro:include filename="$(find mybot_description)/urdf/materials.xacro" />
 <xacro:include filename="$(find mybot_description)/urdf/macros.xacro" />


    <link name="footprint" />

<joint name="base_joint" type="fixed">
  <parent link="footprint"/>
  <child link="chassis"/>
</joint>



    <link name='chassis'>
  <collision> 
    <origin xyz="0 0 ${wheelRadius}" rpy="0 0 0"/> 
    <geometry> 
      <box size="${chassisLength} ${chassisWidth} ${chassisHeight}"/> 
    </geometry> 
  </collision>
  <visual> 
    <origin xyz="0 0 ${wheelRadius}" rpy="0 0 0"/> 
    <geometry> 
      <box size="${chassisLength} ${chassisWidth} ${chassisHeight}"/> 
    </geometry> 
    <material name="orange"/>
  </visual>
  <inertial> 
    <origin xyz="0 0 ${wheelRadius}" rpy="0 0 0"/> 
    <mass value="${chassisMass}"/> 
    <box_inertia m="${chassisMass}" x="${chassisLength}" y="${chassisWidth}" z="${chassisHeight}"/>
  </inertial>
</link>



</robot>

Thanks for your help!