If you add a payment link like I mentioned, you just have to enter the URL, like I explained.
If you want to add a PayPal button, like this https://www.google.com/search?q=paypal+button&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjM7JOrw_3TAhWn6oMKHT6iDTkQ_AUICigB&biw=1920&bih=950#imgrc=CWfcD6qopn3YvM:
Then you need to modify the file located in /inc/widgets/pricing_tables_widget.php
However, several lines will need to be modified to allow HTML code to be added.
This would be considered custom work, which unfortunately at this time we do not offer.
However, you can easily hire a WordPress developer to do this for you quite easily.
I hope that answers your questions.
The easiest solution would be to use the payment link, instead of payment button.