Ring Tones iPhone

This is probably a very basic question, but hey..Im over 50.
I have purchased a ring tone for the iStore, downloaded it to iTunes, transferred it to my iPhone....now what? I can see it sitting there in the music folder..but how do I get to use it as a Ring Tone?? It doesnt show in the Ring Tones in the iPhone..only the bog standard iPhone ring tones are there.
Any help gratefully received.
Ta
David
Intel Core 2 Quad Windows XP Pro