There are two ways of having the code completion in Eclipse.
This method involves adding the jQuery library to the project. You can do the same for every library you want (e.g. motools).
You'll soon discover that's not so good because it doesn't work with $. or $().
NB: If you don't see 'Javscript' in the property of the project, that's only because the project was not initially created with javascript support option.
You can verify this point taking a look a the last items of the project (under you files): you probably won't see 'Javascript resources' icon.
In this case you only have to add the javascript support: