I try to enter our payment for college tuition, but it says it is 0$, based upon an answer I gave somewhere. Where can I adjust this answer?

No one can see your tax return or your screen so no one can know  what answer you gave "somewhere" that is messing up your education credit.  Are you by chance filing married filing separately?  If so you cannot get any education credits.  

Go through the entire education interview until you reach a scree titled "Your Education Expenses Summary".  Click edit next to the student's name. That should take you to a screen “Here’s your Education Summary”. Click edit next to the section you want to change; tuition, in your case.