I have this problem from time to time, even in the current Flash Builder 4.5 (I never figured out how to replicate it).
, :
Flash Builder >
ActioScript > > SWC
C:\Program Files\Adobe\Adobe Flash CS5.5\Common\Configuration\ActionScript 3.0\FP9
() FP10, C:\Program Files\Adobe\Adobe Flash CS5.5\Common\Configuration\ActionScript 3.0\FP10