Accessing storyboard items in a tabviewcontroller

I'm in process of producing a Mac version of my Windows app. So far going well.

I've got stuck on what should be something really basic. I've got the main.storyboard file with a NSTabViewController to which I've added a NSTabView and an NSProgressIndicator.

I've got a reference to the NSTabViewController and wish to be able to set the value of the progress indicator however I can't find out what the syntax is to actually reference that item. I'm assuming this is something quite basic so it should be a simple process but a few fruitless google searches later nothing. Hence turning to this resource.

I'm assuming the code should be something along the lines of...

var myProgressBar = mainListTabViewController.something.something;
myProgressBar.DoubleValue = somevalue;

In Xcode it looks like this