As mentioned below, it is currently only available for Aptana Studio 2:
http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi/api_codeassist.htm
Can you privide any information on whether or not ESRI is currently developing a code assist plugin for Aptana Studio 3?
Thanks for the info,
Jason
http://forums.arcgis.com/threads/50971-ESRI-Javascript-Code-Assist-API-2.7-Doesn%E2%80%99t-work-in-A....