The Automatic Text Box option is only in the New Document dialog box itself. Once you click OK there's no going back to it!
I don't have XPress on this computer, so I'm working from memory, but if you look at the New dialog box, there's two columns.
The first lets you specify the size of the document and the number of columns you want (normally this only draws margin guides, but with automatic text boxes, it will also split the text box up into columns).
The second sets the outer margins for the document (which also sets the position for your text box), and underneath it you will see (I think!) 2 check boxes: one for facing pages, and the other for Automatic Text Box. That's the one you want!
Incidently, in all the years I've been using QuarkXPress, I haven't found another way to link text boxes across master pages, even facing master pages. Kinda makes sense, because if you only use one master page, where would the link go? Still, if anyone can think of a way, I'd love to here it!