If you experience problems and you're not sure whether the plugin is loaded, the first thing to try is to set verbose output in gazebo. In the launch file which brings up gazebo (track it down via your top level launch file), you can set the verbose flag to true, e.g.

   <include file="$(find gazebo_ros)/launch/empty_world.launch">                                                    
     <arg name="verbose" value="true" />
     ..

If the plugin library file is not found for some reason, unfortuately the gazebo error is only printed if you set the verbose mode.