Do you have a ringtone app installed? Did you download it yourself from the web?
Either way it'll be on the SD card, and in order to view the folders, you need a file manager/explorer.
Any ringtone app will have a folder within the root directory of the app for the ringtones... downloaded (from browser) will be in the 'downloads' folder.
You can also manually add ringtones by creating a folder on your SD card called 'ringtones' (all lower case. Same goes for notifications, create a folder, 'notifications' (all lower case). Adding songs/sounds to these folders will allow the file to show up in the selection list for either ringtones or notifications.
EStrongs File Explorer is my recommendation.... or you can also download Astro File Manager, another good app