See this thread for details:
The docs were updated, but there should be some sensible defaults or at least a notification that pops up when you're using an external ivy file, and you haven't done this mapping. Documentation is now good, but people will probably miss this step if there's no warning.
IMPORTANT This project has moved to https://github.com/sbt/sbt. This issue still exists to avoid dead links, but GitHub limitations prevent the original issue from being correctly preserved here. Please see sbt/sbt#214 for the full issue and to comment.