You go to the interest income section in TurboTax.
When you get to this area and the Bank's name is listed from the 1098INT you received, you should see a small box below the bank name that you need to check : My Form has info in more than just box 1.
Then you will see Box 3 to report the US Savings bond interest. If you cashed them in at one bank, you can report the total amount, not each bond separately.
TurboTax will ask you some additional questions regarding the bonds.