Where are the building entrances in Zombies?
I believe the only available answer may be: that there is no certain answer, but the building entrances should be located by player agreement, based on the application of some degree of common sense to the visual appearance of the tiles.

I don't own Zombies, but I remember when I played it I thought it was definitely a "fast and loose" style of game design, rather than having the sort of very precise, loophole-closing ruleset that I personally prefer.  Bad news for us obsessive-compulsive rules lawyers, in other words!