Its an external library that you can use with flash.
Download the source files from this site.
There are 2 ways to use it:
1. SWC can be added to flash
2. Or use the com folder
The com folder is the easier one I think, after you downloaded the package unzip it and find the com folder.
Create a new flash file save it and put the com folder in the same directory.
Then you import greensock into flash code
import com.greensock.*;
To test create a box on the stage and convert it to a movieclip f8, give it an instance name in properties "mc1"
Then add this to your code:
TweenLite.to(mc1, 1, {x:500});
Test the file.
Hope it helps!
p