Open the Contacts application, select a contact, and click Edit in the lower right corner of the window. Two of the fields are for "ringtone" and "text tone." The file you selected in Ringtone Adder should now appear in the list, and you will be able to assign it to your contacts. That's what Ringtone Adder does: it enables you to see your custom ringtones in the list of available ringtones in macOS's Contacts app so you are able to assign them to contacts, or as your default ringtone/text tone.
When I preview the ringtone in Contacts, it doesn't play.
macOS seems to only play previews for tones with a frequency of 48000 Hz (stereo), and that are less than 30 seconds long. However, although you probably can't preview tones if these requirements are not met, they will still play when you receive a call or message from a contact.
Why does Ringtone Adder require OS X Yosemite 10.10 or later?
Older versions of OS X do not allow you to set ringtones at all, and no ringtones reside on the system (there is a difference between ringtones and system sounds).
How can I add tones purchased from iTunes?
When you purchase tones on the iTunes Store, they are designed for iOS, and Apple does not provide a way to add them to your Mac. This is another main benefit of Ringtone Adder. When you first launch the application and click Add, it will ask you to choose a file (or files). If you want to add ringtones purchased from iTunes, by default, these can be found in ~/Music/iTunes/iTunes Media/Tones. The ~ symbol represents the path to your user folder. To get there quickly, in Ringtone Adder's "Choose files" window, press Command + shift + G, and enter the file path mentioned above.
I'm told that my ringtone is too short.
If you still want to add that ringtone, download a program like Audacity and add silence to the end. For whatever reason, macOS will not recognize tones that are less than one second long, so Ringtone Adder won't let you add those.