So you enabled sbt-idea in your global
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