To import an existing Grails project into STS, add the .classpath & .project files that Eclipse requires by running this in the root of your existing project first:
grails integrate-with --eclipse
and then import the project into Eclipse as normal.
Scala IDE – plugin for Eclipse
The Scala IDE plugin for Eclipse is only currently compatible with Eclipse 3.5, and not yet compatible with 3.6.
The update site to point to for the plugin is here: http://download.scala-ide.org/update-current-35