I did a "workaround" as follows:
1. Go to forms view.
2. Find form 1040 wks.
3. Scroll down the form to the entry just above and to the left of the line 12b entry, and right click it. Select Data Source.
4. In the pop-up, click the entry dollar amount.
5. In the new form that appears (Charitable Contributions Summary), right click on the highlighted entry and select override.
6. Change the value to $300 or $600, depending on what the max is for you.
