can't use diffdrive plugin in gazebo

edit

hello everyone , i write a SDF robot model in gazebo and it parse in gazebo successfully , then i attach a diffdrive plugin in it , want to let it sub cmd_vel , but it doesn't work and info in terminal si below :

[ INFO] [1421032451.332067902, 0.023000000]: waitForService: Service [/gazebo/set_physics_properties] is now available.
[ INFO] [1421032451.394642639, 0.083000000]: Physics dynamic reconfigure ready.
[ INFO] [1421032459.012947590, 7.137000000]: DiffDrive missing <robotNamespace>, defaults is my_robot
[ INFO] [1421032459.013117505, 7.137000000]: Starting plugin DiffDrive(ns = my_robot/)!
[ WARN] [1421032459.013202154, 7.137000000]: DiffDrive(ns = my_robot/): missing <rosDebugLevel> default is na
[ INFO] [1421032459.015342813, 7.137000000]: DiffDrive(ns = my_robot/): <tf_prefix> = my_robot
[ WARN] [1421032459.015596405, 7.137000000]: DiffDrive(ns = my_robot/): missing <publishWheelTF> default is false
[ WARN] [1421032459.015621822, 7.137000000]: DiffDrive(ns = my_robot/): missing <publishWheelJointState> default is false
[ WARN] [1421032459.015752576, 7.137000000]: DiffDrive(ns = my_robot/): missing <wheelAcceleration> default is 0
[ WARN] [1421032459.015774787, 7.137000000]: DiffDrive(ns = my_robot/): missing <wheelTorque> default is 5
[ WARN] [1421032459.016082552, 7.137000000]: DiffDrive(ns = my_robot/): missing <odometrySource> default is 1
[ WARN] [1421032459.016282714, 7.137000000]: GazeboRosDiffDrive Plugin (ns = ) missing <publishTf>, defaults to 1
[ INFO] [1421032459.017624231, 7.137000000]: DiffDrive(ns = my_robot/): Try to subscribe to cmd_vel!
[ INFO] [1421032459.023184965, 7.137000000]: DiffDrive(ns = my_robot/): Subscribe to cmd_vel!
[ INFO] [1421032459.025141207, 7.137000000]: DiffDrive(ns = my_robot/): Advertise odom on odom !

if anyone seen this before , how can make it work?