The multiSensorPluginTest.world is located in /home/user/src/GazeboPlugin/world folder. This world file includes the Gazebo plugin for co-simulation with Simulink using the following lines in its .xml body:
<plugin name="GazeboPlugin" filename="lib/libGazeboCoSimPlugin.so"><portNumber>14581</portNumber></plugin>
You can find this line in the .xml body of the world file.
And the export command is a MATLAB command, The filename field must be pointing to the location of the compiled Gazebo plugin. This path can be relative to the location Gazebo itself is launched, or you could add it to the Gazebo plugin search path by running:
open_system is again a MATLAB function to open a model, library, subsystem, or block dialog box. Make sure the model is in the MATLAB path.