I guess I am lucky. My wife was pretty well instantly interested when I told her how hot I thought it was. I started with my tongue, and then a finger, then 2. Then I got her some small plugs and a set of dilators. Eventually we got to medium sized plugs, and she really loves the ones with a tail. Bouncing around the house w/ a little white rabbit tail poking out from under a mini skirt, its really sexy.
My advice would be to talk to her about it, let her know how much it excites you, ask her if she will try. Maybe you find out that her first boyfriend tried to shove his cock in her asshole dry and she is really frightened of it, or maybe you find out that she's been wanting you to play with her bottom the whole time she's known you and she thought you were some sort of prude. You can never tell until you just talk to her and let her know how you feel. If she tells you that she is scared it will hurt, convince her to let you show her that if you go slow and you are gentle, it wont hurt (and then go slow and be gentle).
If she will try, don't mess it up going too fast, be gentle, use your tongue and your fingers at first. Remember that you don't have to get a plug in there on the first day, or the third or the 20th for that matter, its most important that she enjoys it. If you hurt her butt, she will never let you near it again. When you are using your fingers, definitely lick her pussy and make her feel good. Another thing that might help is if you let her play with your ass if she wants to. It will help you understand what she is experiencing and maybe help her understand what you find so erotic about it. If you find the idea letting her play with your ass off-putting, consider that it is just as "unnatural" for her as it is for you. I agree with WildOrchid, if she sees the first plug and its black and huge and shiny, its gonna be scary for her, whereas if its soft and pink and pretty small, she wont be as scared.
So, to sum up, talk to her about it, let her do it to you first, get a nice looking small plug at first, go slow, be gentle, listen to her, and above all don't hurt her butt.