Heading north on the I15 I find it easiest to take exit 42B - Phoenix-downtown las vegas (93/95 south), then stay in the right handed lane and get off on exit 75B - Downtown LV - Casino Ctr Blvd.
This route will take you past the MSS and end up by the old bus transportation station on Stewart Ave. From there you go west (turn right) on stewart and end up right up to MSS (past binions and the Cal).
Some people get lost though, if you miss exit 42B you can go anywhere so some people get off on the charleston exit and then turn onto Main Street and drive all the way north to the MSS.
If you do take the 42B and 75B exits then you might want to use google maps to see the street (using the street view feature) so you can familiarize yourself with the exits.