I'm not sure which version you are using, nor exactly which tool you want to use for Litho simulation. The WORKbench GUI has moved a number of things around over the last two years; and the instructions for doing a Litho simulation using miniOPC, dense OPC, and my venue, sparse OPC all have different requirements.
A good place to start, if you are on a newer version, would be the "Quick Start to Calibre Tools" (calbr_quickstart.pdf) in the doc package. It walks through the basics of the most commonly used tools. It went public only a year or year and a half ago. If you are on an older version, you can download it from SupportNet; even with the GUI changes, it may help locate the missing step.
If you happen to be using sparse OPC (OPCpro), my predecessor wrote a walkthrough that is available in the Calibre OPCpro User's Guide (calbr_opcpro_user.pdf). It runs in batch mode and uses WORKbench to look at results, if I recall correctly.
Also, your local Mentor AE or TME should be able to arrange for training for the particular combination of tools and version that you are using -- it should get you up to speed much more quickly than our generic instructions using older technology.