First you must execute the following command:
source ~/catkin_ws/devel/setup.bash
In order not to execute the above command each time you open a terminal you can add it at the end of your ~/.bashrc
file.
To execute the code open a console and run:
roslaunch art_autonomous_exploration everything.launch
This will launch the simulator, the code and rviz.
In order to actually see the simulator's GUI execute:
roslaunch stdr_gui stdr_gui.launch