Rosbridge and roslibjs: Unable to load the manifest for package

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Hello, I'm trying the communication between javascript and Ros.

I use the example code of "github.com/RobotWebTools/roslibjs/blob/develop/examples/simple.html " with these changes:

var cmdVel = new ROSLIB.Topic({
    ros : ros,
    name : '/fromRos2Can',
    messageType : 'ackermann_msgs/AckermannDrive'    
  });

and

  var twist = new ROSLIB.Message({
    speed:1.0,
    acceleration:2.0,
    jerk:3.0,
    steering_angle:4.0,
    steering_angle_velocity:5.2
  });

and I receive these two error message:

[ERROR] [WallTime: 1404116229.210210] [Client 2] [id: advertise:/fromRos2Can:1] advertise: Unable to load the manifest for package ackermann_msgs. Caused by: ackermann_msgs

[ERROR] [WallTime: 1404116229.213060] [Client 2] [id: publish:/fromRos2Can:2] publish: Cannot infer topic type for topic /fromRos2Can as it is not yet advertised

My c++ program works with AckermannDrive.msg and a test with a std_msg/String works too.

Where is my mistake? Thanks