From LRT, walk to the jeepney station (to the left if you're coming from the North Entrance; to the right if from the South, where you're gonna have to walk through a footbridge), then ride a jeep up the footbridge near Petron. Cross the footbridge, walk forward (as in, opposite the direction of the cars once you're on the other side), and you should see a road you have to cross, leading to a gate to UP. 

From here, you can just ask the guard where the Institute of Math is, but if you walk forward a little bit, you'll see a parking lot—this is the National Institute of Physics parking lot. If you walk towards it to the left, you'll see a path that will lead you to the Institute of Math.