It is recommended to run a Batch Update after creating new starters in order to bring everything up to date.
If you have just created a new starter it is possible you will not be able to select any pay scales when adding an addition to basic pay as they are a pending starter.
You can either wait for the employee to be exported to Payroll or follow the steps below:
Go to Setup and click Batch Updates.
Click Clear all user security cache.
Click Save.
Click Reload All Cache and click Save again.
If you are still not seeing any options for pay scales once this has been completed please refer to the following article as this may also be due to the settings on the pay element itself.