You can add an employee's salary on his Basic Pay page.  Go to Employees>Click Employee>Basic Pay.  

Click Set Basic Pay.

The Edit Initial Basic Pay box will pop up.  Enter the employee's Basic Pay and select his pay rate type.  Click Save.

You will see the Basic Pay that you added by going to the employee's Earnings page.

If you would like to edit an employee's Basic Pay, you will need to add a Basic Pay Adjustment.