Problem; duplicating an instance
 

(PS I'm working on Maya 8.5)
I'm trying to model half of a head and decided to duplicate it on the opposite side to make it whole. I went to Edit>Duplicate Special (options), set the X scale to -1 to have an opposite effect, Geometry as Instance and Group Under as Parent. Whenever I push the Apply or the Duplicate Special button, it never does anything. I also tried selecting all of the vertices but that also failed... Here's a desktop image if people have to know the absolute details.
Thank you very much for reading!

Ok first off you need your pivot point to be on the center axis. To move it select the geo and then the move tool then hit insert (ins on the number pad) and snap the pivot point to center by holding ctrl + X and middle mouse button over the center line of the grid , finally hit the insert key again to complete the pivot move .
Once you have the pivot point in the center you can then do your duplicate operation.

Do I have to select the whole Polygon shape in order to duplicate? Since it's still not working, urg...

If you want the whole object to be mirrored over to the other side you have to select the entire object , if you are in component mode just right click on the geometry and slect object mode from the marking menu and select the whole object.
With your pivot point on the center line of the grid duplicate the geometry usind ctrl + D, then in the channel box (top right of the UI ) scale the geometry -1 in the X to mirror it over , then under mesh hit combine and finally select the center vertices and merge them individually.

Thank you VERY much for your time!!! It works now! I truly appreciate it.