Using foreground select tool to create transparent background

[jcamenga]

I want to create a transparent background for a pic using GIMP on my MAC. I duplicate the layer, use the foreground tool to select and fill my image, hit return, then initialize layer mask for selection. When I hit "add", the background does not disappear like I hoped. Attached is the image just before I "add" the layer masks. The "after" shot looks just the same. The background has not disappeared. Any thoughts?