After setting up payroll for T4As, nothing shows up in Box 28 on a printed T4A.
To setup payroll for T4A processing, see T4As in Payroll.
In order to print the T4A slips, go to Payroll Government Reports / T4As. Enter the payment year and select the employees. Click on the 'Print Total Amount' box in the Other Information box.
Then select Box 028 - Other income from the drop down menu.
This will print all of the 'other income' from the earning codes in Box 28.