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1. Is your plate2.stl really in the gazebo_worlds package, and not in your own package (e.g. robot_of_doom_description)? You say it is in the same location as your attached URDF - if this is in your own package, as it should be, the path to the STL file is obviously wrong.
2. Once you have the right path, try to load the URDF in Rviz:
   roslaunch urdf_tutorial display.launch model:=robot_of_doom.urdf

If it works in Rviz, I am not entirely shure what the problem can be, as I am not shure what the next step is in making a working model in Gazebo. But let me know how it goes.