Lecture theatre 257 is on the ground floor of the Kelvin Building, so from outside 222 (the lecture theatre we used Monday-Wednesday) walk down the corridor away from the main building entrance until you reach the corner of the building, turn left and walk across the length of the building until you reach the next corner, turn left again and walk to the end. Lecture theatre 257 is here.
Watch out for the novelty office just before 257!