At this page: https://developers.google.com/java-dev-tools/download-wbpro, there is no option to install WindowBuilder Pro to Eclipse Juno (4.2).
Is WindowBuilder Pro already installed with Eclipse Juno?
Also, is there a better free alternative to WindowBuilder Pro?
WindowBuilder is now included in Eclipse IDE for Java Developers.
eclipse-java-kepler-SR2-win32.zip
WindowBuilder seems to be the best gui designer at the moment, since it can parse and work with any manually written code, it doesn't require any additional files(xmls etc). And it always tries to make the smallest change possible to the code.
If you need to get WindowBuilder on a computer that is connected to the internet, it's easy. "Use Eclipse Marketplace."
If your computer is not connected to the internet (can't access Eclipse Marketplace), you'll have to do something different for Neon and Oxygen:
Using the first link in the old answer, you'll click on the "Integration Version" "link" for the version you're interested in (4.6, 4.7, etc...). On that page you'll be told something like "You're close! This URL is an Eclipse software repository; you must use it in Eclipse (see how)."
However, you can do it yourself by clicking on "Show Directory Contents" (just down a bit and to the left).
Click on 'repository.zip' and download it. Unzip it.
In Eclipse, choose 'Help->Install New Software...' Click 'Add...' in the "Install" dialog. Click 'Local...' in the "Add Repository" dialog and select the unzipped repository folder in the "Browse For Folder" dialog. Click "OK" in these two dialogs and you'll find yourself back in the "Install" window.
You should see "WindowBuilder" in the large white-space. Click "Select All" and "Next >". There may be some problems with SWT. You may end up at a page called the "Install Remediation Page". Click 'Next >'.
On the 'Install Details' page, click 'Next >'.
Agree to the license (click "I accept..."), and click 'Finish'.
You'll need to restart Eclipse.
Done.
UPDATE FEB 2017
As of Neon, it is no longer included: http://www.eclipse.org/downloads/packages/eclipse-ide-java-developers/neon2.
Please check that website for any future updates, as it could change at any time.
Note that you can also easily obtain WindowBuilder from the Eclipse Marketplace.
UPDATE (year 2014)
As Sean Lynch noted in his answer WindowBuilder is included in Eclipse IDE for Java. Actually, it was already included there since version 3.7 (Eclipse Indigo), which was released in year 2011.
Note that currently, it is included by default only in the version for Java Developers, NOT in the version for Java EE Developers, which is the most downloaded and wanted version. So for the Eclipse IDE for Java EE Developers you still need to install it yourself.
Old answer
It is not installed by default in Eclipse.
The new WindowsBuilder Pro home page is here: http://www.eclipse.org/windowbuilder/
On that page, in the "Current Status" section on the right, it says (emphasis mine):
How to install WindowBuilder in Eclipse.
1) Copy the correct link for your version of Eclipse from the Update Site column (Release Version) on the download page:
2) In the Eclipse main menu choose Help > Install new Software...
3) In the appeared window click Add... button:
4) In the appeared dialog box, in the Name field write some descriptive name (like "WindowBuilder Pro") and paste the correct link (see step 1) in the Location field. After that click the OK button:
5) Select all the checkboxes that are going to appear, then click Next button and install WindowBuilder:
Another way to add a link to the list of available Software Sites:
in the Eclipse main menu choose Window > Preferences, in the preferences window choose Install/Update > Available Software Sites, click Add... button and go on as described above.
As for the better alternative, well, seems like WindowBuilder Pro is the most preferred: Best GUI designer for Eclipse?
Hope this helps.