Breakpoint Support in Flex SDK
From FDT Documentation
When source code has been added to a SDK, FDT will then scan for ADocs and integrate the sources to use with breakpoints.
Before getting started adding break points make sure that you properly have the sources attached.
With your sources attached, simple navigate to a component and use 'Open Declaration' (F3).
With the source code open, add a breakpoint and then invoke the debugger.