Retirement tax questions

There's no need for a box 7 entry in TurboTax. 

Box 7 simply reflects the sum of the amounts shown in Boxes 4, 5, and 6, which you should have already input in TurboTax if they were present on the form.

See Box 7 - Total Gross Paid in Explanation of Form RRB-1099-R Tax Statement for more information.