Hillegass: Cocoa Prog 3rd Ed: Ch. 33 on OpenGL: Where's the Matrix?

I'm using XCode 3.1.4, on OSX 10.5, targeting same. In Ch. 33 we're asked to drag an NSOpenGLVi开发者_如何学编程ew and an NSSlider onto the IB library, and then do Layout -> Embed Objects in -> Matrix. Except the Matrix option is always disabled.

What do I have to do to enable the matrix?

I figured it out -- just select the slider, not both items. Once you know that an NSMatrix is for replicating a widget, that makes sense.