To add a class, log in to your UAccess Student Center (http://uaccess.arizona.edu), using your NetID and password. From the main screen, find the Enroll link on the left side of the page; in the Enrollment area, you will find links to Add, Drop, and Swap courses, among other tasks. In the Add tab, you can search in a few ways - see the left side of the screen shot below.
The box labeled "Enter Class Nbr" is looking for the 5 digit code associated with a course. For ECON 200 in the image, it is 53893). You will rarely know this number! It is more typical to use Class Search or My Advisement Report (which is a list of your requirements based on degree(s), major(s), minor(s) declared at any given time).
When you have searched for a course, selecting it will add to your shopping cart. Be sure to click the green "Proceed to Step 2 of 3" button and follow the instructions from there to add the course from your cart to your schedule.
Adding a course to your cart does not reserve a seat for you (just like adding an item to your Amazon shopping cart does not reserve the item for you). You must add the class to your schedule to reserve your seat.