The official site says that the bus stops at the corner of 5th and Dearborn, but that is a nasty five-way intersection, and you don't want to be standing on the wrong corner because it'll take you two weeks to cross the street, and you have to do it while dragging your luggage with you.
Okay, TL;DR because you're in a hurry: The bus stops on 5th Ave next to the Uwajimaya parking lot. Formally: Northbound after the intersection. Here's a picture of a BoltBus at its designated spot.
The bus stop used to be further north, by the International District bus/train station. Here's a picture of the BoltBus at its former stop. In fact, there are more pictures of the bus at its old stop than there are of the bus at its new stop.
The stop moved to 5th and Dearborn last year. The story I heard was that the old location conflicted with the Metro bus stop at the same spot.