The first thing you need to do is to figure out why she's not going (as stated above.. ours was a "who's in charge here" issue.). It could be any number of reasons.
1. Attention
2. Control- this is usually a big one just because small kids aren't really in charge of anything.
3. Outside issues (abuse, divorce, etc.)
4. Fear
5. Etc. (meaning "I know there are more, but I can't think of any") *smile*
Then (as Dr. Phil says..) you need to find the child's "currency". What do they like/not like?
After that, introduce consistency. Don't start to go without diapers for the day and then turn around and re-introduce them the next day. It's very hard. Some days you just don't feel like dealing with cleaning all of the wet/poopy spots around the house, but be tough. Remember that you can get a steam cleaner for the carpets, the couch cusions are, in fact, washable... etc.
Finally, patience. Don't yell, or belittle the kid (it kind of bothered me a little that you told her that only babies use diapers..? But everyone has low points. Try not to do that anymore.)
And try, try try! It might take a week, it might take six months, you may have to keep her home out of daycare for a week or two until she has it.
Good luck! (and don't take anything I say as absolute gospel- just ideas. I am not a doctor, psychologist, or a professional in any way!)
