Why can't they just get along??

For those of you not in the know, I've introduced a kitten to the household as a companion/playmate for Smudge. Who is nine. She lost her playmate and was wandering around the flat crying, ostensibly for her missing friend. So I got Riff. Female kitten. It's been a few months and Smudge hasn't accepted her. There's lots of hissing and spitting and huffing, Smudge has pinned Riff by the throat.
I'm really stressed.
Advice??