1. Clone the Scala plugin Git repository :
2. Modify scala.properties file to set up specific Java, IntelliJ IDEA and Scala SDKs. Make sure, that idea.home points to the last IDEA EAP (may be downloaded from the EAP page) and
Scala distribution has version 2.8 (last nightly builds are available on the official Scala site)
4. Run ant from command line. Directory dist with two .zip-archives in the plugin's project folder will be created. Unzip the content of *-bin.zip to the appropriate IntelliJ plugin directory:
Mac: ~/Library/Application\ Support/IntelliJIDEA90/
5. Restart IntelliJ IDEA.