Go back to the elementals and talk to them again. They only gave me the rune after I'd found the White Witch. The staff disappears after you free the Elementals, that's why you can't find it.
How could you find the White Witch without having the rune? You have to use the rune to get to the WW.
I found a auto-save that I can use to try this again.
Apparently if you have a companion (not one of your two main characters) put the staff on the forge then you don't get a rune after your character talks to the fire elemental. Obviously a bug, but at least now I can avoid it.
