If the term a student needs to register for does not appear in the term drop down on the Student Registration feature in AMOS, it may be related to the student's academic plan. If the term is not on the student's plan, it will not appear in Student Registration.
To check/add the term, go to the "My Academic Plan" page in AMOS. If the term the student needs to register for isn't on the plan, click the Options button, select "Manage My Year/Terms" and add the appropriate term. Once that's done, the term should appear in the Student Registration drop down.