How do I adjust an employee's time when they weren't present at all that day?

You need to apply an "Adjustment" transaction for that day since there are no Checkin/Checkout transactions to modify.

To create an adjustment transaction, go to the Employees Folder. Then go to the Attendance Tab. Now select the MODIFY HOURS button.

This will cause a window to display that allows you to define the adjustment hours. Simply post an adjustment for +X hours for the date in question (X being the number of hours to increase)