Liz,
I just wanted to give you a couple of comments. I can't do pull-ups either, and I fear they are like push-ups. You just have to do them to get better. But, that said, I still feel your frustration if you can't even do one.
I don't know if you have P90X, but they show several modifications with the pull-ups, including the use of a chair. Instead of using a chair, I use the high step with 4-5 risers, and try to only put one foot on it.
Another suggestion that was made by Kathryn on this forum is to get your chin above the bar and slowly go down (a negative pull-up). I hope I described that right!
I definitely feel your pain, and will keep trying them.
Good luck, Sandra