As 42 indicates, if you have the test gear, then I recommend you acquire the FET drain current and voltage waveforms, with sufficient resolution about the switching transitions. One crude method is to then calculate Pd = Vd x Id for small sections of that waveform, and then sum them up over one cycle, then multiply by the frequency to determine a constant power dissipation. That may be easier for you than setting up a simulation, and then aligning the simulation waveforms to your measured waveforms, and getting the software to do that integration. If you post some sections of the waveform then the forum can assist. Some waveform sections within a cycle will have a relatively constant dissipation, such as when the FET is fully on and fully off, and so are easy to calculate.