as said, museum is right next to the main gate, but le mans is quite a small place, you won't miss it, though it may be sign posted circuit du mans 24 heures if my memory serves me correct as opposed to le mans....

try and check out the village arnage whilst your out there, thats where all the petrol heads go the night before the 24 hour race and all the lttle cafe's are stuffed with hundreds of pics of all the hot motors, again, its a tiny village, but worth the run through and a wander and a drink