Only the blue ball is selectable and can be activated in blender 2.8 and 2.81, or is there something i am missing?

Notice how the "Disable in Viewports"  (the screen icon) and the "Disable in Render" (the camera icon) are purpel? That means there is a driver controlling these. While in pose mode take a look at the 'N' panel; there should be a Rig UI tab. In the Rig UI you will have access to the control that changes the ball shapes.

I have seen the RigUI, that is solved. Now, how do i fix the visibility issue.

Please, i need it to be able to continue.

https://cgcookie.com/lesson/multiball-rig-demo?wtime=99

This will allow you to switch ball types.

Thanks a lot. works