After looking at examples of spline joints I'm even more confused. Which
direction is the spline supposed to go in? If looking at a miter joint,
say, in a box, does the spline go in from the top of the box "down", or from
the side of the box "in"?

For example, look at the splines on these two pages:

http://www.smokymountainwoodworks.co...nd/Pic0110.jpg

http://www.woodezine.com/06_2004/06_...s/figure_2.JPG

It would seem to me a biscuit would be more like the spline in the first
photo? Is this what I need for this application?