Go to the professor IN PERSON. Show up to the first day of class if necessary. It is very easy to turn down a student via email. Go in person (either on the first day of class or before) and give the professor your sob story about why you really really need this class. Of course you'll annoy the professor, but he/she may likely give in. I know I do. The college at which I teach has a major problem with demand outpacing the number of available seats. So each semester I have a boatload of students trying to get into my class. The ones that email, I can easily write back and say the class is full. If you come in person and take no for an answer immediately, you will get turned away. If you give me some story about how you have to graduate this semester and absolutely have to have this section in order to graduate or it is your dying grandmother's last wish for you to take the class right now or some such thing....I will often (not always) give in and sign the add slip. Also, make sure that you talk to the professor individually and not in front of a group of others also trying to get into the course.