I was able to get into it through a back door. I edited the "Custom" file in the project config folder. I copied and pasted the equation from the oil pressure sensor into the tank level sensor. After restarting TunerStudio it showed up with the grayed out advanced radio button checked. So now I can go in and edit it properly. I figured I'd post this info in case any one else is trying to find out how to enable it. Perhaps it's a bug in TS?
The code I pasted in wasn't valid, but it was an easy change once I reloaded TunerStudio. I was missing a comma between the expression and the "Tank"