What are you trying to model exactly? MCNP is used to model the transport of neutrons and gamma's, but it isn't the right tool to model radioactive decay.
Are you trying to calculate the thermal source? If so, it is a fairly easy hand calculation, you just need to know the the mass of Pu-238, the half-life, and the amount of energy released per decay. You might also have to include the decay of any daughter products from the decay of Pu-238.
If you want to actually calculate a temperature distribution, you would probably need to use a finite-element code that performed heat transfer. You would enter the heat source (calculated by the decay of Pu-238), and then the code could calculate the temperatures of the surrounding medium, including the thermal couples.