Follow the steps below to enter the  amount on Form 1099-R (Box 2a) that is treated as ordinary income :

1) Click the Federal Taxes tab and then click Income.

2) Click the Start (or Update) button to the right of "Other income or loss".

3) On the Other Income page, enter "1099-R Box 2a" as the description (see first screenshot).

4) Enter the amount treated as ordinary income in Box 2a under "Amount" and click the Continue button.

5) Go into Forms Mode (click the icon in the upper right side of screen). 

6) Under Forms in My Return (upper left side of screen), look for Form 1041 and click that form.

7) Ensure that the Box 2a amount appears on Form 1041, Line 8, Other income. (see second screenshot).