The trip from Zürich Airport to the closest Italian city, Chiasso, is 76.00 Swiss Franc one way without discount in 2nd class. I would suggest that you buy this ticket separately and then buy a one country Interrailpass for Italy. From Chiasso to Milan you will then use the Interrailpass.

Thank you for the great answer.  

IF this is the only trip In IT, then simply buy a add-on single Chiasso (=border point) to Mil. Do not waste a passday for this-its just 50 kms. Its even cheaper when you detrain there and use local regional train-less as 10€. 

You could use a Globalpass

Or you look for cheap fares on sbb.ch for Zürich Flughafen to Milano

And travel within Italy with a Italy One Countrypass (offer of Eurail) or a Trenitaliapass (offer of Trenitalia)