While in LFR on the "Ultraxion" fight a button to activate "Heroic Will" is apprently meant to appear on the screen.

This does not happen on my UI.

As a work around I use a macro:

/stopcasting
/click ExtraActionButton1

However, I am curious as to why the "ExtraActionButton1" does not display on my interface?

The extra actionbar is weird, basically, since we have no events to determine when its actually shown. The best I've managed is to implement a hack to detect if the bar should be shown, and if so, then show the corresponding Dominos version of it. I think this is working, but I've not tested it fully in combat at the moment, still.