Steps in adding older Flex SDK to Flash Builder 4.6.
1. Download the Flex SDK from sourceforge.net here.
2. Extract the SDK zip package to the Adobe Flash Builder SDKs folder. Conventionally, it is located in C:\Program Files\Adobe\Adobe Flash Builder 4.6\sdks\ for Windows.
3. On the FB 4.6 IDE, go to “Windows” and open up “Preferences”. On the left pane, expand the “Flash Builder” menu, and select “Installed Flex SDKs”. Now on the right, click “Add” and browse for the SDK folder you copied in step 3. A name for this SDK will be automatically displayed.
4. Now you should have new SDK listed as Installed Flex SDKs. Check the newly added SDK and click Apply.