NOTE 1: Workforce uses the term Cost Centres for worked hours to be costed against certain codes. In the clock's menu, these are called Work Codes.
NOTE 2: Before Cost Centres can be used when clocking, they must first have been set up in your Time and Attendance application. Click on this link to see how to do that - Workforce TNA - Cost Centres, General : Aussie Time Sheets - Workforce TNA
There are 2 options for using Cost Centres on time clocks - optional and mandatory.
This setup is to set Cost Centres on the time clock as optional.
To see how to make it mandatory, click here - Helpdesk : Aussie Time Sheets (freshdesk.com)
1. Press the M/OK to enter the Menu.
If you the menu is locked, someone with Super Admin privileges will need to unlock the menu before you can proceed.
2. In the Menu, go to Personanlize
Then Shortcut Key Mappings 
Select "ESC/[-> Key"
Then set the function to Work Code.
Finally, press the ESC key several times until you get back to the blue home screen.
3. On the blue screen you should now be able to press the ESC button which will bring a list of cost centres that were created in your WorkforceTNA (Click here - Helpdesk : Aussie Time Sheets (freshdesk.com) - if you need to create Cost Centres).
4. Once a cost centre has been selected you will need to clock-in/out in the usual lway.