To quickly add a PayPal button to your site, go to Add Content > Popular and drag Single Donation Left or Single Donation Right onto your page.
Double-click the button to add your merchant information.
Once your merchant information is confirmed you'll be asked to enter a donation cause (optional) and set a donation amount. This is the amount that will appear on the PayPal checkout page when a visitor clicks the published button.
Click Save Changes.
You can also change any button on your site into a PayPal donate button.
To do so, click the button text to launch the formatting toolbar, then click the hyperlink icon. From the hyperlink options menu, simply choose Paypal Donation and enter your merchant information (if necessary) and donation amount as above.
This is how your donate button will appear on your site: