Assuming that you are running a local (as opposed to network) application, and are using RSLinx Enterprise for your server, it's simple. In the communication setup, you should have a 1789-A17 backplane. Right click on it and choose Add Device, then pick ControlLogix 55xx emulator from the list. You'll be prompted for the version, name, and slot number of the emulator. I don't think that the name is critical, but the version and slot number must be correct.