Installing 3.6 & 4.5 SDKs
From FDT Documentation
When upgrading to or installing FDT 4.5, you'll need to make sure you have the appropriate Adobe Flex SDKs installed. Below is a video that will walk you through downloading and adding the SDKs to FDT.
We also have a general SDK Management tutorial available as well. Check that out if you'd like to learn more about using SDKs with FDT.