Android

I know how to add ringtones and select them for the default ringtone. However, I want to add a custom ringtone to specific contacts. Yes, I know to go to the contact and click on the menu to select a ringtone. However, the only ringtones I see are in the Preset folder. I can't find that folder when I connect to my PC via USB. The other option is Sound, but it's empty. Does anyone know how to access those folders to add different ringtones?