You can either:
1) use dropbox or box(get the free 50GB offer) and then transfer them to either the notifications or ringtones folder using a file manager(such as astro)
or
2) plug in your usb and select the Media sync (MTP) option from the pop dialgue box on your phone. You will then see the contents of your phone as if it's a removable drive on your pc, so you can then just place them into the ringtones or notifications folder.
After they're placed in the notifications or ringotones folder they'll show up automaticlly when you go to select an audio file