When you leave the theatre, turn right onto King Street West. Make another right at John Street and then make another right at Pearl Street. Walk a few feet and you'll see the stage door on your right. The link will show you what the stage door looks like.
The cast will typically leave through this door. Make sure you know who you're looking for, as cast members in touring shows (during stops in Toronto) may not be expecting anyone looking for autographs/photographs.
I walked past the stage door after going to Book of Mormon and there was no one there waiting to get autographs or anything, and the cast were all heading out very quickly after the show ended. In keeping with what PastorErnst said, it didn't seem like anyone was expecting any stage door-ers, so get there fast and know who you're looking for.