If you want an employee to appear on the diary then when adding the employee you must select 'appear on diary'.
If you have added an engineer to the system already, but didn't select 'appear on diary' then you can change the employee status by clicking on 'Your Business' and clicking on 'Employee'. This will bring up a list of all the employees.
Click on the employees name to bring up their details. Click on 'appear on diary'. Select 'yes' and click save. The employee will now appear on the diary.