mike,
It's generally not a good idea to book train tickets for premium trains on the day of flight arrival. If your flight is late or any other delays occur and you miss the train you've booked, your tickets are worthless and you'll likely have to buy more at full price. For arrival at MXP it's just as easy to buy tickets at the airport rail station after arrival. It's best not to pre-purchase in this case. If you buy at the airport tickets will cost a bit more as you'll be paying "Base" fare, but it's generally a safer method to use.
I assume your flight arrives at MXP Terminal 1? If that's the case, use the Malpensa Express to Milano Centrale. Note that there are two different destinations with the Malpensa Express. Be sure to board the one to Centrale and NOT Cadorna. Also, DON'T forget to validate your ticket prior to boarding the M.E.
Could you clarify where you'll be travelling to from Milano Centrale?