Via Paolo Sarpi is the main street that makes up the area that's known as Milan's Chinatown. It's to the north-west of the city centre, about 10 minutes on foot from Arco della Pace. When I have a craving for Chinese food I head for zona Sarpi. There's a couple of places I can recommend; one's where I usually go to, it's a little place on a sidestreet, Trattoria Hua Cheng on Via Giordano Bruno, no. 13. I normally pay around 10 euro for three different dishes and a bottle of water. Jubin on Via Sarpi itself (no.11) is almost the complete opposite as it's a spacious restaurant, and it also offers Japanese and Thailandese food as well as Chinese.
Sarpi isn't the easiest place to get to, for one thing the street's now pedestrianised (though being Italy you'll obviously see the odd car driving down the street every so often...). The nearest metro stop is Moscova on the green line, and it takes around 10 minutes on foot to get to the nearest end of the street. The no.43 bus stops at the western end of the street, and the no.12 and no.14 trams (both of which also stop at the Duomo, Cardusio, and Lanza metro stops) stop streets that cross Via Sarpi. To walk from Arco della Pace, head down Corso Sempione and take Via Pina on the right. However you get there you'll know you're in the right place from the Italian flags hanging in the street!