The option to add funds is unfortunately not available in every country. If you go to transfer below your balance and there is only the withdraw option then you won't be able to add funds.
As a buyer you never need funds in your Paypal balance. Paypal will automatically take payment from any confirmed bank account or card (whichever is eligible in your country) during the payment process.
If you have the option to select a funding source then you can do so here...
Click on the profile tab (icon next to log out) > Click on payments third option along on the thick blue band top of page > second down click on update next to your preferred way to pay and put a dot in one of the options listed.
Advice is voluntary. Kudos / Solution appreciated.