In the cell cards, you specify the temperature in units of MeV by multiplying the temperature in Kelvin by the Boltzmann constant 8.617e-11 MeV/K. For example, 600K would be "tmp=5.17020E-08"
However, this is not the only place you need to worry about temperatures. In the material cards, you need to attach the library generated at the correct temperatures. A list of the library identifiers and temperatures used to be included in the "Appendix G" of the users manual, but it has now moved to a separate document. There may be a newer version, but I have report LA-UR-17-20709.
If you are using the ENDF/B-VII.1 cross sections, ".80c" is at 293.6 K, ".81c" is at 600 K, ".82c" is at 900 K, etc.
Finally, you also need to make sure you are using the correct temperature S(alpha,beta) cards. These can be found in Appendix G also.
If you need temperatures that do not line up with the library temperatures, there is a utility program "makxsf" that you can use to interpolate existing libraries and create libraries at user specified temperatures.
There is also development work to allow "on-the-fly" temperatures, but I am not sure what the status of this is. It will probably be released in a future version of MCNP.
