1) The correct locations for storing custom ringtones and notifications are:

/sdcard/Ringtones
/sdcard/Notifications

2) You must copy the files to this location from your computer via USB. The problem I was encountering was caused by the fact that I was using ES File Explorer to download my MP3 and OGG files from a network share to my phone. While all other applications were able to see and play these files without any trouble, the built-in ringtone/notification chooser will not see files copied onto the device in that way.