I currently have a job AS an administrative assistant while I'm applying. I'm a secretary to a surgeon - and he evaluates quite a few medical students. If you are speaking to her (assuming female, I could be wrong), first name is probably ok. If you are writing to her, use Ms.
The really important thing to remember though, is that I ALWAYS tell my boss about the med students, etc. So if you are genuine, polite, and helpful, I tell him how great I think you are. If you are rude, arrogant, or talk about him behind his back (yes, it's happened), then he knows that too.
The cool thing I learned about this job is that the secretaries can make or break you. It's also funny how the doctors depend on the secretaries to tell them things like that. 🙂
So just err on the side of polite and you'll be fine!