Doors and Windows - Help with replacement windows!!

i am replacing my old single-pane DH window with new vinyl replacement windows. I was able to get the old window out and the new one fits fine while doing a test fit- One questions though (yes- I've read the instructions several times and am still confused)- There is a lone vinyl piece called a "sill adapter"- Where exactly does it go?? Under the window?? I've read and called the manufacturer (have not heard back)- and I cannot see what the purpose of it is?? Can anyone help who has installed these?

I'm going to take a wild guess.... It's L shaped? and it should go under the window. DH windows have a slope at the sill and this sill piece should take up the extra outside space of the window for a cleaner look and support.

Glasman is right and here is an instruction sheet:

http://www.lowes.com/lkn?action=pg&p=Improve/RepWindow.html&rn=RightNavFiles/rightNavEnergy#term (http://)