It'd be really nice to know how to unlock classes as well. I really don't like playing blind.
On the World Tab, the different areas you have available to explore, some of them have '!Class' to the right of the name, somewhere within that world it looks like there is a chance of opening up one of the classes when exploring one of the 4 smaller zones inside. The name of the class seems to somewhat coincide with the name of the area, like I found the necromancer class in the the necromancer's lair under the Aurora Citadel (lvl 100), then the '!class' label went away. Not sure of they all coincide like that, but at least the ones I remember finding do.