From the girl side of this, I'd like to add my two cents. My boyfriend and I were together for over a year before he told me. It was something new to me (I'd actually never even heard of it before), but I immediately got into it because I love turning my boyfriend on. To be honest, I had noticed something missing in our sex life, and once he told me, I was glad I could fill that need. Generally, you just have to work on it in steps. For instance, when my boyfriend and I first started with diaper play, he would just wear them around the house under pajama pants. Then we worked up to him wearing them without pants, then to me changing him, then to him wetting in them and me changing him. Now we just got locking pants (which are awesome, btw).
The thing I cannot stress enough is that you HAVE TO GIVE HER DIRECTION. I realize that a lot of ab/dl is about relinquishing power, but before you do that, you have to give your partner a heads up about what you want. Be specific: do you want to call your partner mommy, master? How long do you want to be in a dirty diaper? Do you want to be tied up, and if so, for how long? Spanking, baby talk, gags, anal play? Tell her as much as you can, and she'll be less nervous.