Where can I enter the $2400 I contributed to my grandsons' 529 account?

I'm nor familiar with the SC soft ware, in particular, but here's the generic answer:

 

There is no federal deduction for making a contribution to a 529 plan. Some states allow a deduction on the state return. If your state is one, it will come up in the state interview; usually in the "Here's how (your state) handles income differently" section. For a list, see:

http://www.savingforcollege.com/compare_529_plans/index.php?page=compare_plan_questions&plan_question_ids[]=437&order_by=plan_question_437

In your South Carolina return look for the screen, Here's the income that South Carolina handles differently.  The Education section should be right below Retirement.  [See Screenshot #1, below]

 

If you do not see the Education section there, look up under the screen title.  If you've opened the Education screen before it may appear at the top, for easy reference.  [Screenshot #2]

 

Screenshot #1

 

Screenshot #2