Basically what I had to do was delete half of the apple I made and then use a mirror modifier. So I did that but I just got two halves of the apple. They weren’t joined.
Easiest way in this case is to select clipping in the mirror modifier settings. Then in edit mode select all vertices and drag along the X axis so it overlaps the mirrored mesh. Stop pressing with your mouse then drag back again. You may have to unselect clipping again if some verts are clipped to the centre that shouldn’t be there. http://www.screencast.com/users/blenderwho/folders/Jing/media/a836b12f-97b6-4d94-bb0b-c248f0c23bcf
so select all vertex on edges and scale X (or Y or Z ) 0 to align in straight line
then select one vertex on the edge then Alt-S and select cursor at location then space bar to search " origin"
and select new center cursor to have the center at that vertex
and then you can apply the mirror to it and it should be aligned at the center of the apple