See DoninGA's info in his latest comment above, which includes an image from the desktop version. Or if for some reason you can't get that screen, see if you can make the selection that you want this way:
Go into Forms Mode. Pull up the Federal Information Worksheet (Info Wks) in the left column list. Scroll way down to "Part VI--Additional Information for your Federal return." Under "Standard Deduction/Itemized Deductions" see if you can indicate your preference with the boxes there.
