What do you mean the tool is not active? Do you mean levels are not associated to views? In other words, the head are black; not blue. Then, go to Plan Views on the View Tab, pick Floor Plans and then select the Level you want to activate as a view. Also, if your creating new Levels using the tool on the Arch. Tab, take note of your Options Bar settings. If you want a Plan View to be associated with the Level, you need to check that box.
@Anonymous hello, maybe you are in the wrong view, if you were in plan view the level is not active, try to go to elevation and section and you will see the level will become active. thanks