In Studio 3 much of this sort of functionality is still coming as Aptana is going more of a TextMate bundle route for a great deal of their features, which I think is really cool.
That said, there is already a jQuery bundle. The jQuery bundle, if you don’t have it installed yet, can be installed from Aptana via the Commands menu (Commands->Bundle Development->Install Bundle and then select jQuery from the list).
This support discussion has some details about jQuery code assist in Aptana:
https://aptanastudio.tenderapp.com/discussions/problems/2006-studio-3-jquery-support-is-lacking
In particular, it mentions adding a jquery vs doc file (http://code.jquery.com/jquery-1.4.1-vsdoc.js) to your project. This seems to be the key.
If you install the jQuery bundle and add the aforementioned JS file, you can get jQuery code hints in your Aptana editor(s). I just ran through this in one of my Rails projects and it does work (I’m on the latest beta of Studio 3 for Mac).