Malpensa Airport TIM store?

I think there are none. I drove to Gallarate to get to a phone store in June ( Vodafone).
I also checked the Tim website and none appear at the airport proper, just in town at Gallarate.
The TIM website gives me a location of a Tim store on piazza duca d'aosta, which is where the central station is (not sure if it's inside or outside the station).

There is a TIM phone inside the Milano Centrale station, I bought a phone there in May. Have the salesperson set it up for you to default to English. Make sure your plan will work in Switzerland.

Zoe gave you the answer. The TIM store is inside the station.
Not sure you need a SIM card for a dumb phone or a smart phone.
If you have a smart phone, TIM offers the following:
1Gb data + 600 min voice + 600 SMS (messaging) for €19 for one month.
If you can do with only 500 Mb, they offer the following under the "TIM Special" offer:
500Mb+200 min voice + 200 SMS for €15 for one month.
They also have a weekly plan for €10 a week. It includes 500Mb + unlimited Voice and SMS.
To the above you need to add €10, which is the cost of the SIM card.
The full list of offers is in the website below (Italian only):
http://www.tim.it/tariffe/chiamate-sms-internet?tab=1

While in Switzerland, you will be roaming. For that purpose you have the following options:
TIM IN VIAGGIO PASS: 500 Mb+250 Min Voice +250 SMS for €10 for a week
or
TIM IN VIAGGIO FULL: 25 Mb/day + Voice at €0.16/min + SMS at €0.16/message for €3 per day.
You would select the one that fits your planned time in Switzerland.
Full details for out of Italy roaming are below. You need to activate the "Data Roaming" on your smart phone when you use it.
http://www.tim.it/tariffe/chiamate-sms-internet?tab=1

To activate any of the plans TIM IN VIAGGIO the details are below (click on "Come si attiva" tab)
http://www.tim.it/estero/tim-in-viaggio-pass

The TIM store will give you the details, but I suggest you read it (using Google Translate).
Once you activate it, as you cross the border, TIM will automatically deduct the charge from your credit. You need to have some credit in your account in order to navigate, so in addition to the plan above you need to add at least another €5 to your SIM card. If, for whatever reason (for example if you call overseas from Italy, which is not included in your minutes) your credit goes down to zero, your data plan also stops to operate. You will need to add some money to the SIM card so that you have some credit, to reinstate the plan. To add credit to your SIM card, the easiest way is to go to a TABACCHI bar (Tobacconist), who can do so online in a second. Tabacchi bars are everywhere and are a sort of 'field' agencies that have everything besides tobacco and spirits (postage stamps, postcards, gov't stamps, telephone cards, State lottery tickets, etc.). The Tabacchi bars are your friends and you will certainly use one, so learn how to spot them. This is the symbol at the entrance of a Tabacchi bar/store:
http://imganuncios.mitula.net/vendita_bar_tabacchi_ricevitoria_a_napoli_99972330020450074.jpg

Thank you so much for that information! Very helpful!!!

Follow up question: will the Milano Centrale TIM store have a nano SIM for the iphone 5? Other posts about cell phone sim cards have mentioned the nano size is not always available.