You can choose whether employees can enter Time Off requests or not.
You can turn on and off the TIME OFF tab for the entire account by going to SETTINGS > Global Employee Permissions. (This impacts the account as a whole, and is not directed at specific employees.)
Looking at that page, you can be sure the option 16 “Can Enter Time Off” is set to “With Manager Approval” or “At Will” to allow them to access a TIME OFF tab from their employee interface.
The next time employees log in using their own username/password, they will see TIME OFF in the top menu and they can click that to view their time off and request time off.