Federal review asks "Form 1116 (COPY 1) -- Foreign Tax Credit Comp Wks: Line 4b3b/4b3c Total shouldn't be less than or equal to zero," what do these correlate to?

Two questions: (1) I accidentally entered '0' for 4b3b and now I can't get back to edit it again. Can I go back to it somehow to change it? and (2) What nonzero value(s) does it want me to enter into these 2 boxes?