Tip Adjustment is available if your payment gateway is configured to take tips.
To enable Tip on POS you go into Configuration - Settings - Payment Gateway and check the Tip adjustment box, then the POS will allow the merchant to add Tip or Adjust Tip for credit card transactions.
Once credit card transaction go through they are posted on the search screen. If you want to Add Tip or Adjust Tip you select the "Tip Adjustment" button.
Then a popup from your Gateway comes up and you will be able to Add Tip or Adjust Tip. Once you enter the Tip amount you select "OK" and that will add the Tip to the transaction on the gateway.
You can also bring up the Tip Amount Processor by pressing "ALT+N" together from the transaction screen.
Once the popup comes up you enter the credit transaction number and it will bring up the transaction amount and you will enter the Tip amount and press "OK". That will add the Tip to the transaction on the gateway.