Question

I have an iMac running El Capitan and using Studio One 3 Artist.  I have a firewire interface, but I do not have a Z button to select in Studio One.

Answer 1

It depends on the firewire interface that you have.  The FirePod is a very oldschool firewire interface and was great in its time, but it does not support the Z latency feature.  

The Zero Latency feature is ONLY available in Studio One and works with the AudioBox 1818VSL, FireStudio Project/Mobile, and the Studio 192.

You need to create 2 or more output busses in Audio I/O Setup and check the box for Cue Mix.  Then in the Console Mixer view, you can click a Z button for the Cue Mix of any audio channel.  The Z will be located just to the left of the padlock button for the Cue Mix.

Answer 2

This was happening to me for a long time as well.  I went back through some tutorials and while navigating through the Audio Setup, I corrected it.  In Options, under Audio Setup, I had my Dropout Protection set to Minimum.  As soon as I updated that to Medium, the Z latency feature was available.  Hope this helps!