Для этого необходимо правильно установить путь к классам - show managedClasspath для конфигураций compile и runtime, чтобы проверить его:
[sbt-13-0-1]> help dependencyClasspath
The classpath consisting of internal and external, managed and unmanaged dependencies.
[sbt-13-0-1]> show managedClasspath
[info] List(Attributed(/Users/jacek/.sbt/boot/scala-2.10.3/lib/scala-library.jar))
[success] Total time: 0 s, completed Jan 1, 2014 12:10:57 AM
[sbt-13-0-1]> show runtime:managedClasspath
[info] List(Attributed(/Users/jacek/.sbt/boot/scala-2.10.3/lib/scala-library.jar), Attributed(/Users/jacek/.ivy2/cache/org.docbook/docbook-xml/jars/docbook-xml-4.4.jar))
[success] Total time: 0 s, completed Jan 1, 2014 12:11:01 AM