Is it not in the Gradient group of tools...?
It should be hiding under the gradient tool. But Shift+Backspace (or delete) will bring up the fill dialog that allows you to choose black/white/gray/foreground/background etc. Or Alt+Backspace for the foreground and Ctrl+Backspace for the background (I might have those backwards).
You might be talking about the Paint Bucket Tool, just below the Eraser Tool. Shift - g brings it out of hiding.
It's not the same thing as the Fill dialog which you are bringing up.