Pegasus still uses RAMPS, right? You ran 12V to the 11-amp inputs on RAMPS?

I assume you've already wired up the SSR. The MakerFarm website says the Pegasus heat bed has an integrated relay. You've jumpered across the relay contacts, right?

The LCD reads room temperature for the heat bed? If it is showing MINTEMP or MAXTEMP, Marlin likely won't let you turn the heater on until you fix the thermistor problem.