Sorry, but I don't understand your question.
Why are you looking for a Z button?
What are you trying to do (that you can't find in the manual)?
There is a settings page for dropout protection, but it has nothing to do with the Mixer. The mixer is just a visual representation of the controls for your tracks.
Drop-out settings are part of the audio engine and how it communicates with your sound card/interface.
On Windows: Ctrl Comma will take you to the settings page with Dropout protection.
Hope this helps.