I had the same problem. Your AS3 class path is pointing at a directory to low in the flashdevelop library folder structure to pick the mx class prototypes.
1.) Within Flashdevelop press F10 to open the 'Settings' window.
2.) In the list on the left click on the 'AS3Context' option.
3.) Imediately Under the group title of 'Language' on the right hand side of the window, find the 'AS3 Classpath' option. Its value is typically set to 'Library\AS3\Intrinsic'. Change the the 'AS3 Classpath' value to read 'Library\AS3'.
4.) FIXED. Flashdevelop should now be aware of the mx libraries, thus autocomplete should now also work.
Library\AS3\frameworks\Flex4
as well as forspark.components
– Bisson