What version of Dreamweaver are you using? In MX/MX2004/8 it is in the same place as the Windows version: Cmd + J to open the dialog box and it's under 'background image.' If you're us Dreamweaver CS3 I can't really help you since I haven't used that version yet and don't know if they've moved things around.
Odd. I'm using Dreamweaver on my Mac, but it looks like the Windows option you showed. Yours looks like Dreamweaver a few versions back.
It's a preference. Tables/CSS, something like that. I remember seeing it before, but at work and don't recall the actual setting.