It’s very useful to have Flex framework source available in your workspace, so you can easily search it, and so that Flex framework source files can be opened with Ctrl-Shift-F. (They can be opened with Ctrl-Shift-T even if the files aren’t in the workspace.)
1. In your workspace, create a new project: right click in the navigator, pick New > Project…
2. In the wizard dialog that appears, under General, pick the plain “Project” choice and press the “Next” button.
3. Enter a name for the project. I use “Flex Source”. Leave “default location” checked, and press the “Finish” button.
4. After the project is created, right-click on the top level of the project and pick New > Folder.
5. Don’t fill in the Folder name, but instead click the “Advanced” button, check the “Link to folder in file system” checkbox, and enter the path to the “mx” folder underneath the framework source. On my system it’s in C:\Program Files\Adobe\Flex Builder 3\sdks\3.0.0\frameworks\source\mx. This should auto-fill the folder name field, and it’s fine to accept the default. Press the “Finish” button.