I assume you have an extract for that at the moment, correct?
If yes - what's your goal?
Do you want to simply see the Custom SQL? (#1)
Or you'd like to modify it and recreate extract? (#2)
If #1 then you can do following:
a) Feed it any file (or two files) to get to Data Source tab
b) Use "Reset ignored messages" from Help menu and reopen workbook - it'll show Custom SQL warning
If #2 then you will need to find source files to be able to refresh an extract. Or do 1a and remove file connections and continue with just Custom SQL.