The Script.aculo.us Toolkit runtime does not come shipped with the the Eclipse Script.aculo.us plugins. The Script.aculo.us toolkit must be download from the Script.aculo.us site at http://script.aculo.us/ and configured to the ATF infrastructure. Download the Script.aculo.us Toolkit and unzip the toolkit into a directory.
Open the ATF Preference page for defining Installed AJAX Runtimes, Select Window > Preferences... > ATF > Installed AJAX Runtimes
Click the "Add.." button to add a runtime.
Select the Script.aculo.us Toolkit from the "Runtime Type" drop down menu. If the Script.aculo.us runtime is not in the drop down menu, the base runtime plugins for that Script.aculo.us haven't been installed into eclipse. Example implementations of different runtime plugins are available from the ATF site on eclipse (www.eclipse.org/atf).
Fill in the "New AJAX Runtime Instance"
Click the "OK" button. Make the newly define runtime the default version of the runtime used by ATF. Select the newly define runtime on the "Installed AJAX Runtimes" preference page and press the "Set as default" button. Multiple Script.aculo.us runtimes can be defined and ATF will use the one that is set as default.