The Problem.
Well the problem you’re facing is pretty simple, but it can also be tricky. I had the same problem some time ago, and it pretty much ruined my day. (mainly because I rush and I don’t read the documentation, which is BAD by the way)
The Solving.
There are several reasons for which you will get an error message when trying to import a component , but if you have the feeling that you did everything right and it sill doesn’t work, this may help you lot. So, don’t forget that you have to actually drag an instance of the component on the scene from the component’s panel (Window->Components to open it) no matter if you import the component in actionscript. You can delete the component from the stage afterwards but the important thing is to have an instance on your library.