Gazebo plugin-Failed to resolve RegisterPlugin error+ Gazebo topic visualization

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BH

I'm trying to attach a plugin to a launch file. When launching the file, I keep getting this error: Error [Plugin.hh:158] Failed to resolve RegisterPlugin: /home/hayim/buildworldtutorial/build/libwall1world.so: undefined symbol: RegisterPlugin[ INFO] [1431075019.325996696]: Starting plugin Kobuki(ns = //)!

Must mention,The plugin is attached according to the [Overview of Gazebo Plugins tutorial] while the names are changed and also the helloworld.cc file.

Generally, the code should move the turtlebot forward and I wish to see that in gazebo.

By the way, how come I can't see the cmd_vel_mux topic by picking the topic visualization tab on Gazebo?