It is difficult to be sure with such a limited extract of your build definition, but my guess would be you are in a scala build file and didn't import the dist key in scope.
Try adding the following import to your build file
import com.typesafe.sbt.packager.universal.UniversalKeys.dist
addArtifact has the following signature :
def addArtifact(a : sbt.Artifact, taskDef : sbt.TaskKey[java.io.File])
UniversalKeys.dist is defined as follows:
val dist = TaskKey[File]("dist", "Creates the distribution packages.")
So the types are correct at least :)