Wendy,
Ever since sublayers were introduced in AI 9, the way to create a mask that spans multiple layers has been to use sublayers and layer masks.
First you make a containing layer that includes all the layers you want to mask as sublayers. There are many ways to do this. Usually what I do is select all the layers that I want to become sublayers in the Layers palette, and then use the "Collect in New Layer" command from the palette menu. Then I draw the masking path on the top of that containing layer. Many people seem to be averse to palette menus and prefer using buttons and drag-and-drop. If that's you, you can draw your mask on a layer that is above the other layers, and then select the other layers, and drag them into the layer with the masking path.
Whatever, once you get the sublayer structure with the soon-to-be-a-mask path or text on the top of that layer, you select the layer that has the mask (in the Layers palette, not on the artboard - the buttons on the Layers palette ignore the artboard selection), and click on the Make/Release Clipping Mask button at the bottom of the Layers palette.
Mordy Golding has an illustrated step-by-step on his blog site about layer masks.
<http://rwillustrator.blogspot.com/2006/01/beauty-behind-mask.html>
Object masks such as are made by the Object>Clipping Mask>Make command are more for when you have a single object such as an image that you want to put in a frame and always have the frame select and move with the contents.