I haven't used Unity for a while (since Unity 2) and have just come back to find that a lot of things just don't work the way they used to. In particular, I can't find the menu item for terrain lightmapping anywhere!
I do have a directional light in my scene, but it doesn't seem to help in any way.
I've also noticed that there is now a lightmapping option that I don't remember within the settings for the directional light, but it doesn't seem to change anything no matter what I set it to.
Select your terrain in the hierarchy. Make sure static is selected on the terrain in the object tab (it will probably be static already) and choose your terrain's lightmap resolution from the drop down. If you are not sure what your terrain resolution is, go to Terrain>Set Resolution.
Add a light to the scene, make sure Bake Mode is set to Auto (the default) or Bake Only.
If you want the light to cast shadows in the lightmap, you have to enable shadows on the light - otherwise it won't cast any. Setting a shadow type for a light will alert you that Pro is needed, however this is not true for light baking, and it will still function correctly with the indie version of Unity 3.
Select bake. Your lightmap will appear in the preview section in the lightmapping window.
(I'm adding this as an answer because 'Add Comment' doesn't work in my browser - Firefox)
OK, I think I've found a clue. It looks like the support documentation I found was just lies. (This platform is confusing enough to use without leaving stuff like this up in the support area! Grrr.)
I have managed to find - through Google searches - that there is now a Window > Lightmapping area. Unfortunately, I still can't get it to do anything to my terrain. Nothing comes up in the Preview pane no matter what I select, and if I go to the trouble of baking a lightmapping it makes no difference at all to my scene.
All I want to do is set up a lightmapped terrain!
Please help! I can't believe it's this hidden and this complex.