Hi Tuongvan,
Every school will probably have slight variations on what they call things. If you have to apply for admission to a program, you'll deal with admissions. Otherwise, only with registration. But they still might have the registrar's office. Without meaning to be too silly, I'd call the person who works in registration "The person who works in registration." I had to do that once. "Hi - I just spoke to a lady who works in registration, and I said I would call back with my membership number, but I didn't catch her name." "Oh yes, that was Sharon. Let me put you through to her."