You can enroll in the math classes for which you placed once your placement score has posted to UAccess. You can check to see if it has by logging in with your NetID and password here: http://math.arizona.edu/academics/placement/aleks_scores.html. If there are no seats available for the math class you want to enroll in, please keep checking back on UAccess each day, as many people drop the course and sometimes more seats are added.
Do NOT wait until your letter grade has posted. It is your placement score which grants you access to your next class.