You can add a simple donation (or elaborate!) by using the HTML Block in the pagebuilder.
You'll need to go to Paypal first and get the code for the button:
Go to "Profile", then "Selling Preferences", then "My saved Buttons".
They have a few default button types for example, and you'll want to create a new button. It will ask for its type, select "donate" and enter your information.
Generate the code, and copy it.
In pagebuilder, select the page you want this box to end up.
Drag a "html block" up to your page, and begin entering the information.
In order for the button code to show properly, you need to switch to html mode view first, (HTML) then paste the button code you saved into the section.
Switch back to regular edit mode and add your text/message or whatever.
Simple as that. Save it and its ready to go.
Additional Notes:
The button looks best when centered.
You can add additional feature by adding calls to other scripts, like levels page, a thank you page, etc. theres many php/java based freeware donation modifications out there that you can use as an example, it just takes a little work.
Enjoy!