Camera : how to implement camera to the world?

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Hi I am doing project with two Universal robots - UR5 and one mobile robot - Smart. I do not know how to implement the camera as urdf to my world. Could You help me? I attach screen of my scene.

I would like to upload a camera to this world, but I don't know how to do it. I tried to change the code from the tutorial: http://gazebosim.org/tutorials?tut=ro... but I still have errors in the terminal. Besides, the examples in the tutorials do not show how to insert the camera itself, but how to attach it to a robot, for example. Could someone of you explain me step by step how to do that? I can show you my launch of world and all the urdfs and codes. Thank You for help! :)