Go to Employee Maintenance (KE0001) -> Enter an Employee Number -> Tab -> Click the Payroll tab, shown below highlighted in yellow.
The payroll tab will launch screen (KE0043) -> Select the box below 'Active' -> There should be a 'Y' in this box, as shown below highlighted in yellow.
Double-Click the 'Y' (or use the binocular icon), this will launch screen KE0042 -> Here you can enter the Job Code.