Er, for 3d meshes, you have to save the tool as a .ZTL file. It doesn't save the 3d mesh if you just save the document - you have to save the 3d model from the Tool palette (the Save Tool As button, should be just above the Export/Import buttons).
So I started doodling on a sphere, then saved teh document as a .zbr file. I come back a day later wanting to work on it some more, open up the file, my previous work is right there on the viewport, but how the hell do I select the mesh so I can sculpt it again? (I know, this is probably a dumb question and I am totally…