Can't get ContainPlugin to trigger in Tutorial

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Hello, I can't get this ContainPlugin to trigger the light on according to this tutorial (http://gazebosim.org/tutorials?tut=co...).

I've downloaded the files to ~/gaztut/contain_plugin/,

then ran $ cmake ../ $ make in ~/gaztut/contain_plugin/build/,

set the source with $ usr/share/gazebo/setup.sh,

added the library path with $ export GAZEBO_PLUGIN_PATH=${GAZEBO_PLUGIN_PATH}:~/gaztut/contain_plugin/build,

then started $ gazebo ~/gaztut/contain_plugin/contain_example.world --verbose.

The ball rolls down and away but no light is triggered on. Shell shows:

Gazebo multi-robot simulator, version 9.8.0
Copyright (C) 2012 Open Source Robotics Foundation.
Released under the Apache 2 License.
http://gazebosim.org

[Msg] Waiting for master.
[Msg] Connected to gazebo master @ http://127.0.0.1:11345
[Msg] Publicized address: 134.130.76.57
[Wrn] [ColladaLoader.cc:1422] emission texture not supported
Service [/~/contain_example/enable] is not valid.
Topic [/~/contain_example/contain] is not valid.
[Msg] Started contain plugin [~/contain_example]
[Msg] Loading Example plugin

Are these two messages

Service [/~/contain_example/enable] is not valid.
Topic [/~/contain_example/contain] is not valid.

relevant? I've tried another world-file only launching <plugin name="TestPI" filename="libContainPlugin.so"> but they still appear.

Further, what's the difference between ContainPlugin and OccupiedEvent from SimEventsPlugin? They both seem to trigger when something enters their specified area... Please help me! Thank you in advance! :)