I can't get resetsim to work in Sims Medieval either.
If it's only your sim who freezes it may relate to his/her action queue. When my Sim freezes I often find that my Sim has to wait for another sim to do stuff. Once my Monarch froze up at the Message Post and wouldn't post his letter. It turns out he was waiting for the Spy sim who was blocked in the Reception Hall and going round and round in circles. I cancelled my King's action and send him elsewhere. That worked.
Also if you've overloaded his inventory, that can slow and freeze Sims. Limit your personal inventory and pass the unwanted items onto the appropriate hero or store them in craft stations, forges, food cupboards etc.