So you enabled sbt-idea in your global ~/.sbt/plugins/build.sbt with:

resolvers += "Sonatype snapshots" at "http://oss.sonatype.org/content/repositories/snapshots/"

addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.3.0-SNAPSHOT")%

If you use Play 2.1.0, then be careful, because play adds a sbt version, too.

The new 1.3.0 of sbt-idea version has gen-idea as command, whereas the play version (1.1.0) has only idea as command. 1.3.0 usually downloads sources automatically, but it doesn't in the play context. In play, you can use idea with-sources=yes.