The tutorials assume a certain level of experience with other ROS packages, so if you don't have that, it's easy to get confused.
After installing all the necessary files on the controller, you'll need to start the appropriate launch file on the ROS PC. For your IRB 120, that would be robot_interface_download_irb120_3_58.launch which is provided by the abb_irb120_support package.
So after building your workspace with the abb and abb_experimental repositories cloned into it, you source /path/to/your/workspace/devel/setup.bash and then:
roslaunch abb_irb120_support robot_interface_download_irb120_3_58.launch robot_ip:=<IP_OF_YOUR_ROBOT>
where you replace <IP_OF_YOUR_ROBOT> with the actual IP of your IRC5 controller.
If that is successful you should now have a few topics publishing information (general robot/controller status and a JointState topic) and a new action server of the control_msgs/FollowJointTrajectory type.
This last one you can send trajectories to and the driver will start executing them.
A popular choice for software that can generate such trajectories for you is MoveIt and for the IRB 120 a MoveIt configuration package is provided in the abb_experimental repository. You should be able to start it -- after having shutdown the previous launchfile -- with:
roslaunch abb_irb120_moveit_config moveit_planning_execution.launch robot_ip:=<IP_OF_YOUR_ROBOT>
where again you replace <IP_OF_YOUR_ROBOT> with the actual IP of your IRC5 controller.
At this point you can plan and execute motions with MoveIt.
Please always keep robot safety in mind especially if you run things in auto mode.
Finally, see Working with ROS-Industrial Robot Support Packages for more information on how to use ROS-Industrial robot support packages and see wiki/abb_driver for more information about the nodes in abb_driver and which information they publish and subscribe.
