In my experience Eclipse works a bit differently in that you don't set a single source but rather multiple sources for different components you want to install. For example, if you install an Android emulator, it will have a different source than a GUI editor like Jigloo. The Eclipse Marketplace
can be a decent starting point to find things though.
By the way, in my opinion, GUI editors are a mixed bag. Yeah, you can lay out an interface easily, but the code those things generate is pretty incomprehensible and really prone to weird layout issues. After struggling with GUI editors for a bit, I decided they are more trouble than they are worth and forced myself to learn how to use layout managers.