You can register for a class up until one day before the semester begins.  If a class begins later in the semester, you can register up until one day before that class begins.  If you miss either deadline, you will need to attend the class on the first day and ask the instructor if you can be added to the class.  If the instructor agrees, he/she will provide you with an add code that will let you register online up until the last date you can add in to that particular class.  Important add, drop, and refund dates for a particular class can be found in the Class Schedule by clicking on the blue highlighted live link under the Dates tab of each course section.