Grasp a box with youbot

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Hello

I am currently trying to grasp several small boxes with the youbot. I have set it up with MoveIt, but I cannot figure out how to give it valid a valid pose. I am not concerning myself with proper object recognition(via sensors), so I am trying to get the object position from Gazebo and give it to MoveIt to move the arm next to the box. Currently I am getting the position of the box using the GetModelState service call. But if I feed the pose it returns me to moveit it once again tells me that I have specified a wrong pose. Here is the code:

robot = moveit_commander.RobotCommander()
scene = moveit_commander.PlanningSceneInterface()       
arm = moveit_commander.MoveGroupCommander("arm")    
def listener():
    rospy.init_node('listener',anonymous=True)
    rospy.wait_for_service("/gazebo/get_model_state")
    try:
        gms = rospy.ServiceProxy("/gazebo/get_model_state",GetModelState)
        response = gms("box","youbot::arm_link_5")
        youbot_node.plan(response)
    except rospy.ServiceException, e:
        print "Service call failsed: %s"%e

def plan(targetPose):
    pose_target = geometry_msgs.msg.PoseStamped()
    pose_target.header.stamp = rospy.Time.now()
    pose_target.header.frame_id = "/odom"
    pose_target.pose = targetPose.pose
    print "============================="
    print pose_target
    arm.go(pose_target)

So any idea what I should do to move my arm to the proper position, and then grasp the box?