The suggestion by support was correct but the terminology used in Creo (and pro/e) has always been misleading.
In an assembly, you can create a mirror placement of a current part by selected the create component icon, then selecting mirror in the COMPONENT CREATE dialog box. You have to give it a name but it won't end up being used. Once you select OK, you get a second dialog box and you select the MIRROR PLACEMENT ONLY radio button, then select the part you want to mirror and the plane you want to mirror about. You can use the preview at the bottom of the dialog to see what it will look like.
It's really misleading because you have to give a new name even though you are actually only mirroring an existing part.
have fun
Steve