When using the Desktop software....a $0 in a field that won't let you change it, usually indicates that a "Supporting Details" sheet is attached....you used it last year and it transferred into this year's file, but empty of any $$ amount.
So....double-click on the $0 number and the S.D. sheet will pop up and you can make your entry there.....or can delete the sheet entirely (once it pops up) to get access to the normal field.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*