You have to download and install the Revit plug-in from Navis. You can download it from their website. If they have one for Revit Systems, once you install it, you'll get a new line on your 'Tools' menu called 'external tools', and that's where you'll find the 'export to Navis' option. It's not built into Revit, it's an add-on.
If they (Navis) don't have a plug-in for Systems, I know they have one for Revit Building that works (we use it at my day job). You could probably edit it's .ini files and get it working for Systems instead by changing the pathing...