To add "Buy Now" and other PayPal buttons to your Tendenci website, you will first need to obtain the HTML code that includes your PayPal merchant account information.
PayPal has online tutorials that explain how to do this here: https://www.paypal.com/cgi-bin/webscr?cmd=p/xcl/rec/singleitem-intro-outside. Click on the "Demo" button to view the step-by-step tutorial.
Follow these instructions and you'll obtain the HTML code from PayPal to embed on your Tendenci website page.
Navigate to the Page you want to add your PayPal button to on your Tendenci CMS website and select to "edit" the page. Inside the WYSIWYG editor on your page, select the "HTML" icon to edit the HTML code of your page.
Insert the HTML code you copied from PayPal into your website page where you want to place the button, then click the "Insert" button on the HTML editor to update the code.
Now, save your page and you are done.