In Windowed Mode, change the game's video resolution to match your desktop's video resolution and NWN will switch to Full-Screen mode. However, there appears to be no way to switch to Windowed Mode from Full-Screen Mode at this time. Other quirks of windowed mode on the Mac include an inability to move the window or hide the window.
You can minimize the window to the Dock by using the widget or double-clicking the window's Title Bar. You can also use Expos to reveal areas beneath the window. Alt-Tab is also useful for switching applications (especially if you have the option to rotate the camera when the mouse is at the edge of the screen turned on).
One workaround for some of these quirks would be to run NWN "full-screen" in a window of a virtual machine or something similar. See methods one and two under 3.09 of this FAQ for ideas on accomplishing this. More.