You must have DESIGN Modeler. It is another program within Ansys Workbench. You are using Ansys Workbench Simulation. You need Ansys WorkBench Design Modeler.
I'm using Design Modeler. I've attached a screen capture of the "tools menu". The only item that is close to what I want is the "face split" option. However, this only splits a 2d face, not a 3d volume.
Gah, sorry to have steered you in the wrong direction. I didn't have ANSYS open when I answered the question, obviously. The SLICE command is, indeed, located under CREATE > SLICE. I hope that I didn't cost you too much time digging around in the wrong place.