This is one of the fundamentally retarded issues with ZBrush's basic design. What you're probably experiencing is that your model is being "dropped to the canvas." This happens because ZBrush is actually just a "2.5D" painting app with an ability to "model" what it considers "tools." So a "tool" can = a paint brush or a 3D object that you can apply to the 2D canvas. Once a model/Tool is dropped to the canvas, it no longer exists as a 3D object. It becomes part of the "2.5D" painting that's always there on your screen whether or not you plan to use it (or care about such a feature, which, IMO, most ZBrush users don't). This is why you cannot use the Edit button. There's nothing to edit any more.Originally Posted by saut
What you need to do is look on your Tools palette to see if the last state of your model is stored there. You might want to clear the canvas before you start working on the "Tool" again, though. When you wish to save your models, you must save it as a Tool, in the Tool palette, not as a document.
Surely other posters can give you more details, but this is one of the things i hate most about ZBrush. It's just not built to BE a 3D modeler. Proper behavior for this kind of thing is in Sculptris...