What's a sum score?
When you take the ACT more than once, the NCAA lets you keep the best score you made on each section from each time you took the test.
Think of it this way: Suppose you did great on the math section the first time you took the test. You took it again a few months later and scored an awesome score on reading this time, but not so good on the math section.
You get to keep your great math score from the first test, and your awesome reading score from the second attempt. It's like banking your best score. The NCAA Clearinghouse won't even consider your math score that wasn't so good.
To get your sum score simply add up the best score from each section of the test. EXCEPT WRITING! The NCAA is not currently counting the writing sections from either test.