Many Flex, Adobe AIR, Adobe BlazeDS, and Adobe LiveCycle ES developers choose to use the Eclipse plug-in configuration of Flex Builder so that they can develop Java code in the same IDE that they use to develop the MXML and ActionScript code. While the standalone version of Flex Builder does not contain tools to edit Java code by default, you can install them as Eclipse plugins. That lets you use the standalone version of Flex Builder to edit Java code.
To install the Java development tools in the standalone version of Flex Builder:
1. Use the Help > Software Updates > Find and Install menu command to open the Install/Update dialog box
2. Select Search for new features to install.
3. Click Next.
4. In the results, choose Europa Discovery Site.
5. Click Finish.
6. Select the Java Development package to install.
7. Click Next.
8. Accept the license.
9. Click Finish.
Note: You might be prompted to install additional plugins required by the Java Development package.
To change perspective:
1. Use the Window > Perspective > Other to access all perspectives.
You can also click the Open Perspective button in the upper-right corner of the workbench window, then select a perspective from the pop-up menu.
2. Select Java from the list of perspectives.

Is there any advantage or benefit to go this route, instead of just installing the FlexBuilder plugin into your eclipse?
Just curios...
Thanks
rmx
reply to rmx:
No real advantage. People who do more design than coding may prefer the standalone version since it is a single install. People who already have Eclipse configured and do not want to change IDEs can use the plugin version.
Stephen
This is good news,
i'm new to eclipse and find it difficult to use and configure (used IntelliJ IDEA before). Since installing flex plugin for eclipse didn't work for me, i'm trying the other way around.
I just need the IDE to write and debug code, for the build part we use maven or ant to have an indipendet build regardless the IDE you use.
So i'd definitely chose the standalone Flex Builder.
Sorry, these directions do not work for me. When I select the "Java Development" package to install I get this error (I think it's because my Flex Builder is running on Eclipse 3.3.1 and not Eclipse 3.3.2. but that's just my guess):
Current configuration contains errors that are not corrected by the requested operation and more errors would be introduced. See details for more information.
----- Current configuration problems -----
Resulting configuration does not contain the platform.
----- Configuration problems after the operation -----
Resulting configuration does not contain the platform.
Eclipse Java Development Tools (3.3.2.r33x_r20080129-7o7jE7_EDhYDiyVEnjb1pFd7ZGD7) requires feature "org.eclipse.platform (3.3.0)", or compatible.
Eclipse Plug-in Development Environment (3.3.3.r33x_r20080129-7N7M5DQVIA_6oJsEFkEL) requires feature "org.eclipse.platform (3.0.0)", or compatible.
JST Common Core (2.0.0.v200706041905-1007w311817231426) requires plug-in "org.eclipse.core.resources (3.2.0)", or compatible.
JST Server UI (2.0.2.v200802150100-77-CT9yJXEkuiKVeQrclqTHQ3648) requires plug-in "org.eclipse.core.runtime (3.2.0)", or compatible.
JST Server Core (2.0.2.v200802150100-56CQ9rC7EK_UAblGAD2537) requires plug-in "org.eclipse.core.runtime (3.2.0)", or compatible.
JST Server Adapters (2.0.2.v200802150100-787KE8iDUUEF6GwKwpHEQ) requires plug-in "org.eclipse.core.runtime (3.2.0)", or compatible.
JST Web UI (2.0.2.v200802150100-7B1DzCkuNa_RPevwkwB1iJ6z-0RH) requires plug-in "org.eclipse.ui (3.2.0)", or compatible.
JST Web Core (2.0.300.v200802200333-7Q7AE7JEHhHeh0hI5y-QGB) requires plug-in "org.eclipse.core.runtime (3.2.0)", or compatible.
JST Enterprise UI (2.0.2.v200802150100-7b7_Es8EU6AXOV9QLJSees1SQoYQ) requires plug-in "org.eclipse.wst.command.env (1.0.101)", or equivalent.
JST Enterprise Core (2.0.2.v200802150100-42Au_kE77c7EAgE7I7) requires plug-in "org.eclipse.wst.common.frameworks (1.1.0)", or equivalent.
WTP Web Page Editor (1.0.1.v200709110605-3-29oA55P5V8A) requires plug-in "org.eclipse.core.runtime (3.2.0)", or compatible.
Java Persistence API Tools (1.0.2.v200802140100-77-7_CYQCD2CaLYCHCD) requires plug-in "org.eclipse.core.runtime".
AspectJ Development Tools (AJDT) (1.5.0.200706070619) requires plug-in "org.eclipse.core.runtime (3.2.0)", or compatible.
I already have an IDE for Java development and installing the plugin was all I needed, although it was a huge download. It worked right out of the box.