Click the gear icon at the top for settings. You can change the maximum and minimum number of digits, whether the problem should have carry-overs or not and how many numbers to add. If carry-overs are disabled, only two numbers will be shown and the minimum digits will be ignored for the second number.
The answer is automatically checked as soon as you are done. If the answer is wrong on the first try, you need to click the "Check Answer" button when you are done.