Hi, I could say you have two option: First, if you need to attach a camera on the end-effector of UR5 (or any link ), you need to get the ur_gazebo package and install it from the source, then you need to edit the xacro file of the robot ....../ur_description/urdf/ur5.urdf.xacro
and there you can attach the camera to any link by following the same steps in the SensorPlugin tutorial. i just tried it, you can see screenshot (the weight box represent the camera):.
Second option: you mention that you are working on pick and place application, you might need to create your own environment where you will use the UR5 robot. In that case, you can attach your camera anywhere in the environment following the same steps in the SensorPlugin tutorial.
Hope this can help :)
I'm not sure how to add a camera, but I would start with http://wiki.ros.org/ur_gazebo and make sure you are comfortable with the basic UR5 simulation first.