Actionscript 3.0 Classpath Location (Eg: fl Folder)?

Hi,

I am trying to use Flex Builder 3 to write my Actionscript 3.0 Code however whenever I try to Eg: import fl.transitions.Tween; it does not recognize the fl folder let alone the Tween Class to import.

Where is the default Actionscript 3.0 Classpath so that I can add it to Flex Builder 3’s Source path?

Btw I am using Adobe CS4 / Flex Builder 3.

Cheers,
Ryan