Images from Google Maps
If you are coming from the North via Gilmore Avenue, I would suggest crossing Aurora Boulevard then turning left at 1st Street. Then turn left at Doña M. Hemady, then right at Aurora Boulevard until you see the entrance to Robinson's Magnolia's parking area (near the end of the building).
If you are coming from the East, say Cubao, via Aurora Boulevard, you can go past Robinson's Magnolia then hope you can find a U-turn slot. (It might take a while, but there will be one eventually) and travel back to Robinson's Magnolia until you see the entrance to the parking area. (Refer to the previous paragraph).
Or for the adventurous driver, you can try turning left at Betty Go Belmonte, then right at N. Domingo and follow this road until you reach Doña M. Hemady. Turn right and go along Doña M. Hemady until you reach Aurora Boulevard and follow the directions mentioned earlier.
Hope this helps out drivers trying to get to the elusive Robinson's Magnolia Mall.