There are a number of ways to do this, but the easiest is this:
1) Almost every major cell phone company has an e-mail address associated with the SMS inbox of your cellphone. For my T-Mobile, it's the ten digit phone number @tmomail.net -- 1234567890@tmomail.net -- and when you send to that e-mail address, you get an SMS on your phone with the body text. So step one is to find out what yours is. Open Address Book and add this e-mail address to your own address card (the one marked "Me").
2) Then select the appointment in question, and under alarm in the sidebar, click where it says "Message" or whatever, and select e-mail. Your SMS e-mail address will be visible in the list below.