Take the train to Munich central station, then the suburban railway number 1 (direction "Freising / Flughafen") to "Oberschleißheim". From there it's about 15 minutes walk to the Schleißheim palaces; you can also take bus number 292 (only on workdays) to "Mittenheimer Straße" or "Schloss Lustheim".

The municipality of Oberschleißheim is located in the north of Munich and can be reached via the motorways A 92 (exit "Oberschleißheim") and A 99 (exit "Neuherberg") and also via the roads B 13 and B 471.
In front of the New Palace there are about 120 free parking spaces for cars. Coaches may also be parked here.
Route planners you can find at
http://auto.abacho.de/routenplaner/.