error in running differetial_drive package

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hi everyone

I want to use a package named differential_drive which is written as a base_controller for two differential wheeled robot by Jon Stephan.

As i can not install it by source from my zone, i downloded its zip and extract it to my directory. This package consists of some python files and after compiling no node has been constructed. I think .py files are my nodes and i should use them in the launch file as below:

<launch>
  <node pkg="differential_drive" type="pid_velocity.py" name="lpid_velocity">
       <remap from="wheel" to="lwheel"/>
       <remap from="motor_cmd" to="lmotor_cmd"/>
       <remap from="wheel_vtarget" to="lwheel_vtarget"/>
       <remap from="wheel_vel" to="lwheel_vel"/>
       <rosparam param="Kp">200</rosparam>
       <rosparam param="Ki">200</rosparam>
       <rosparam param="Kd">0</rosparam>
       <rosparam param="out_min">-255</rosparam>
       <rosparam param="out_max">255</rosparam>
       <rosparam param="rate">30</rosparam>
       <rosparam param="timeout_ticks">4</rosparam>
       <rosparam param="rolling_pts">5</rosparam>
  </node>
  <node pkg="differential_drive" type="pid_velocity.py" name="rpid_velocity">
       <remap from="wheel" to="rwheel"/>
       <remap from="motor_cmd" to="rmotor_cmd"/>
       <remap from="wheel_vtarget" to="rwheel_vtarget"/>
       <remap from="wheel_vel" to="rwheel_vel"/>
       <rosparam param="Kp">200</rosparam>
       <rosparam param="Ki">200</rosparam>
       <rosparam param="Kd">0</rosparam>
       <rosparam param="out_min">-255</rosparam>
       <rosparam param="out_max">255</rosparam>
       <rosparam param="rate">30</rosparam>
       <rosparam param="timeout_ticks">4</rosparam>
       <rosparam param="rolling_pts">5</rosparam>
  </node>
  <node pkg="differential_drive" type="virtual_joystick.py" name="virtual_joystick" output="screen"/>
  <node pkg="differential_drive" type="twist_to_motors.py" name="twist_to_motors" output="screen">
      <rosparam param="base_width">0.32</rosparam>
 </node>
</launch>

But when i run my launch file, ROS can not find nodes and i see these errors:

ERROR: cannot launch node of type [differential_drive/pid_velocity.py]: can't locate node [pid_velocity.py] in package [differential_drive]
ERROR: cannot launch node of type [differential_drive/pid_velocity.py]: can't locate node [pid_velocity.py] in package [differential_drive]
ERROR: cannot launch node of type [differential_drive/virtual_joystick.py]: can't locate node [virtual_joystick.py] in package [differential_drive]
ERROR: cannot launch node of type [differential_drive/twist_to_motors.py]: can't locate node [twist_to_motors.py] in package [differential_drive]

Is it because i just extract zip files to my directory? I mean should i use another command to locate this nodes in package?