Is the control button actually showing up as “Control”, or does it show as “Save Gcode”?
I’m running Yosemite and Cura 17.10 and sometimes I’ll see the “control” button but a connection can’t be established. In that case I close the control window, unplug the USB cord, plug it back in, hit control and it connects. This happens after I have turned off the printer and then turned it back on sometime later.
In previous versions of Cura, the control window wouldn’t show if Cura had just been started and no other popup opened. My solution for that was to open the expert settings popup from the expert menu, then close it, then press control. It would then open and continue to open thereafter until Cura was closed (or crashed, which was and is a certainty on any print about 4 hours or longer).
It might be worth going through these silly motions to see if there is some strange combination that makes the current version of Cura work on El Capitan.
Also, you might want to install pronterface ( http://www.pronterface.com/ ) – even if you don’t use it, it spits out all the messages Cura might not show when making a connection.