To enter the income in the correct place,

Click "Income"

Click "T-slips"

Scroll down the "Income Slips" page until you see "Foreign Income."  Select "Foreign Income"

Click "Continue" until you see the "Foreign Slip" screen.

Enter the total amount of the US Social Security income received in the "US social security benefits" field.  If you enter the amount in US Funds, ensure you also include the exchange rate in the "Exchange rate" field.

The software will automatically calculate 15% and include that amount at line 256 of the T1.