Add bumper plugin and read the state

Hello, I'm trying to add a bumper plugin in Gazebo and read the state by the topic in ROS, but I can't ge it. All that I get is add the object and read this: header: seq: 16 stamp: secs: 0 nsecs: 0 frame_id: my_box states: [] Time doesn't change and states is empty. The urdf file is: true10.0collisionbumpermy_box I saw in some answers that I need to add 'my_box_geom' [[link]](http://answers.ros.org/question/29158/how-do-i-use-force-sensor-bumper-sensor-in-gazebo/) , but it doesn't work. I also tried with 'test_bumper' in gazebo_plugins, but neither works. I use ROS Groovy, Gazebo 1.5 and Ubuntu 12.04 (I can't update, but I tried the same in Ros Hydro and...the same result). Could someone say how add it correctly? or if you see any mistake in the file... Thank you!