I'm not sure Robert's entirely off the mark, even if his comments seem harsh to you. Law is, in many ways, a profession of deadlines. Miss the statute of limitations and you could be hit with a malpractice suit.
Having said that, we all make mistakes, and it doesn't mean you shouldn't try to work it out. If you live at all close to the school, I would strongly suggest not waiting until the admissions director returns. Put on some nice clothes (you don't need to wear a suit) and walk in to meet the dean. You can simply remind her that you're the person she spoke with, that you really want to attend the school, and that you appreciate her offer to speak with the admissions director on your behalf. I honestly believe that a five minute, polite, professional, and in-person conversation can be the difference between the dean asking the admissions director "is there anything we can do?" and saying "try to do something."
It may or may not work. Almost for sure, your seat was offered to someone else the day after your deposit deadline. You've got to make them want to add you anyway. At least if you try the in-person approach, you'll know that you did everything you can do.
Best of luck,