Hi,
If it's just a melody line that you're after, and you have a MIDI keyboard (just a cheap one will do), you can use my "Bang and Fix" technique (which I do A LOT).
1. Start a new file in Composer, and select the staff you want to record into.
2. Start recording, and just bang in the rhythms of the notes you want. It's really helpful here if you use the metronome, and if you have the melody solidly in your head, because if you play keyboard as poorly as I do, the pitches will not sound right, and might throw you off a bit til you're used to the technique 
3. When you're done recording, go back and fix the pitches. I do this by clicking on the first note, and singing while I use P + up or down arrow key to adjust the pitch til it matches what I'm singing.
4. Then I use the Right Arrow key to go to the next note, and adjust it approrpriately using P + up or down arrow key.
5. Wash, rinse, and repeat. It's actually pretty quick after you get the hang of it.
Or if you can play keyboard and have a MIDI keyboard (cheap ones are only around $50 new, sometimes less for used ones) you can just record the thing in to Composer and be done with it
ttfn,
Sherry