First of all, I am so sorry that you're going through this. I think sometimes that this disease is harder on the people who love us than on us. You have the perspective and love to see how much your wife is hurting herself. She's probably too wrapped up in it to worry. But it is tough on you- if you need to talk about how you feel, we're here for you too. Not just for her.
Most of us have had many people try to help us through the years. I personally never took to it. I've been badgered, cajoled, handed pamphlets, reasoned with, yelled at...I'm sure you've tried one of two of these with poor results, or you wouldn't be asking strangers for help.
How long has your wife had an eating disorder? How long have you known? How did you find out? The longer she's been keeping this secret, the harder it will be to get through the absolute refusal to talk about it. Same thing with how you found out- the more traumatic and embarassing the situation, the harder it will be for her to talk to you. If she told you herself, I think you have a better chance- it gives you an opening, because she wanted you to know.
Has she ever tried to stop before? Has she given you ANY indication that she dislikes this part of her life? If she has, you can start there. "I know you told me that you hate throwing up..."
From there, it gets even tougher, though. For you, a step like going to a doctor or counselor or even telling you when she feels like throwing up seems like a baby step to start things out. For many of us, it sounds more like apocalypse. This disease takes over your whole life, so any threat to it is interpreted as a threat to your life. I know that sounds like an overreaction, but we tend to be really good at those.
Take it too fast and she'll either get angry and shut down or just stop talking and shut down. Either way, you lose.
Take it really, really, really, really slow. Mention the bulimia- talking about it can take away it's power and the humiliation. Obviosly, don't do it at the company christmas party, but if you're alone and relaxed, ask her to talk. Don't have an agenda to get her to do anything. Just ask her how it feels. Get her to trust you and open up. Honestly try to understand where this comes from, how it began, why it continues. It may take a long time to get past the blockade, but keep trying.
Once she is comfortable with that, ask why it's hard to stop, why getting help is so hard. Let her know that you're not going to leave if she changes. Or if she doesn't. Dude, you've got to be in for the long haul, if you're going to attempt this.
Expect fights. Expect tears. Expect new insecurities you never anticipated. You're in totally unexplored country.
The most important thing, I think, is the unconditional love. Sure, you think that she knows you love her- you married her, right? Yeah, many of us are perpetual self-loathers. If I hate me, why wouldn't you? Let her know, frequently and obviously, that she is the light of your life- with or without bulimia.
Start slow and the rest will come. You are making the effort and obviously want to help her, so congratulations! Keep letting us know how things are going- for both of you.