the GUI designer is only availble when you're creating a new component
To clarify a bit, in the Introjucer, with the Files tab selected, right click on the Source folder, and click "Add new GUI Component". It's not enough to say "Add new Component Class..." (either option) if you want to use the GUI Editor within Introjucer.
Hopefully one day IntroJucer will will allow you to layout everything.
Maybe you found this out already, but did you know that you can add a "New Generic Component" and specify the class, including constructor params? This allows you to, say, position a WebBrowserComponent in your design. Of course you don't get a live preview, but it does make positioning things easier.
To this end, the big daddy of the Introjucer is the Projucer, with a release date of TBD:
Note the remark about allowing editing of any valid Component class.
Unfortunately, this is one of those things that jules is either working on in secrecy or hasn't had time for, so who knows when/if we'll see the Projucer in action. Also, there has been some passionate debate over whether such a thing is "good":
What I'm saying is don't get your hopes up about jules adding anything to the Introjucer/Projucer. But if there's a particular component/feature you want, you could add it to the Introjucer yourself since you have the source (sounds daunting, but I'm pretty sure jules has given this answer before).
Like I mentioned in another thread, this is a one man show, and jules gets pulled in 1,000,000 different directions here. It's not quite like other open source projects that have multiple maintainers, but on the bright side, the code is homogenous and fits together well.