I am also trying to make a mod with a parent mod - even having the source to the api does not make it possible to actually run the mod in the development client. As far as I can tell, you need to decompile the mod with the minecraft source, but as Forge does all that for you in one step, I'm not sure how it is *supposed* to be done.
If you have the API, but not the API *source*, you should be able to include the API as an external jar using Eclipe's Project > Properties > Libraries menu. This won't help you with getting your development client working, as below, but it should make it so you have access to the types and so forth provided by the API Jar file.
I have been collecting some information about how to do this at http://www.minecraftforge.net/wiki/Developing_Addons_for_Existing_Mods. Perhaps it will be of help to you. If not, if you figure it out, perhaps you can add to this.
I agree about the state of the documentation, although the Generic Mod documentation was very helpful for getting started. My impression is that the community is still very fractured. Some of the major mod creators tend to look down on people who create mods that use their mods, and others get pissy about it. Then there is BuildCraft! But, I don't need pipes. So.
