There are a few things going on, which aren't at all obvious until you've made the mental connections. It's also complicated by the fact that the NSDocument machinery was invented in the days before storyboards, and so a new superstructure has been retrofitted onto the old understructure.
A document (your NSDocument subclass) has one or more window controllers associated with it. Normally, there's just one, and it's created for you automatically, in "makeWindowControllers". The window controller is a NSWindowController instance, although it can be a subclass of NSWindowController if you wish. If you weren't using a storyboard, you'd almost certainly want a subclass, but because of the way storyboards are organized, you may not need a subclass in your case.
The window controller has a "contentViewController" property which is a reference to a NSViewController subclass. If you look in the storyboard, you'll see a window controller "scene" and a root view controller "scene", with its class set to a custom subclass of NSViewController that was provided for you by the Xcode app template. That's the "content" view controller.
So, you'll want to do the following:
1. In ViewController.swift, create an outlet to the text field if there isn't one already, and connect it from the view controller to the text field in the storyboard scene.
2. In the simplest case, you can set up the text field in the "viewDidLoad" method, or possible "viewWillAppear" or "viewDidAppear", depending on the exact time you want or need.
But there's usually a bit more involved. You'll likely want to follow the MVC pattern, which means your document contains its data model. The data model is created by a new document initially, or read from a document file after it's been saved, using the method you mentioned in your post.
That means you'll likely want your view controller to be able to find the document data model. This is very slightly tricky, because you don't get a direct reference for free. You can, however, get from the view controller to its view, then from its view to the window, then from the window to the window controller, then from the window controller to the document, then from the document to its "model" property (if that's the property you set up when reading in the document data). So, from the view controller (ignoring optionals and type casting):
self.view.window.windowController.document.model
However, you must remember that this chain of references isn't complete untill after the view is added to the window's view hierarchy, which happens after "viewDidLoad". So, this method of initialization must be delayed to (say) "viewWillAppear".
If you ever need to go in the opposite direction, from document to window controller to view controller, you can do it like this (ignoring optionals and type casting):
self.windowControllers [0].contentViewController
You should read the document architecture documentation:
developer.apple.com/library/prerelease/content/documentation/DataManagement/Conceptual/DocBasedAppProgrammingGuideForOSX/Introduction/Introduction.html
but be aware that this was written before there were storyboards on the Mac, and before the major NSViewController changes introduced in 10.10. A lot of the UI related behavior it describes for window controllers is now normally handled by the root/content view controller.