you need to give her more time. when my son transitioned from in home to preschool (2 years old), it took us about 2.5 months to get him to stop crying at drop offs. Try talking it up in a positive way on your way there, let her take in a lovey, a picture of you, something special. Talk to her teachers and get other suggestions. Make sure she is getting enough attention when you leave to help her through this.
Try dropping her off earlier so she's one of the first ones there so she can get more attention from the teachers and see the other little kids come in and get dropped off without crying.
Try bringing a special breakfast item or chocolate milk.
it takes time, but it'll get better. Now I'm having a hard time getting my son to LEAVE pre-school!!! :)