Requirements:
Download FDT
Download Flash Builder Plugin

Now Step by step…

1. Download FDT and install it …

2. Install Flash Builder Plugin in your FDT Folder…

3. Now me MUST change many options because FDT doesnt Support Flex SDK 4 and AIR 2

Here some Screenshots about the configs:

Mandantory Config:
MXML Editor = Flash Builder
AS3 = FDT

Create your Project…

1. switch to Flash Builder Perspective
2. Create Project

3. After Create switch to FDT Perspective

4. Right click on your Project and Flash Project -> Add Flash Nature -> Use Your SDK -> OK

5. Now you can see that your project is a *FDT *project

6. Now you can work with your project with the both nature: FDT and FlashBuilder

IMPORTANT:  Compile it with FlashBuilder if you use newer SDK’s