When you get to Government Center, you'll exit the blue line and go upstairs. Don't take the trolley from the platform immediately to your right...wrong direction. Instead walk straight ahead past the mini dunkin's to the platform behind the main staircase/esacaltor. Walk downhill towards a disused "Pass Sales" area as this is where your trolley will be. As other posts have mentioned, take a trolley marked Lechmere or North Station.