To make the directions aforementioned a little clearer:
From Stand C at Railway Square, take either bus 422, 423, 426 or 428. You'll be looking for something like this with a Stand C sign obviously, instead of D:
Once you're on the bus, it'll go down Broadway and turn left up City Road. When you see an overhead presdestrian bridge. Get off at the bus-stop
(you can see the Carslaw building in the background)
Cross the street, walk through the gates, and when you come round the corner, you should see a building that looks like this:
This is the Carslaw Building, once there, there should be people around that you can ask for directions to the specific lecture theatre, as there are a few in that building.
Good Luck!