How can I get my two cats to get along?

I have a two and a half year old cat named Missy. Recently I got a six week old kitten who I named Shadow. I'm had Shadow for nearly two weeks now and my older cat still isn't warming up to her. Missy hisses and growls and then walks away. Missy used to sleep on my bed but she wont anymore because Shadow's in there. How can I get Missy to accept Shadow?