I don t have ringtone folder iPhone 3G

You need to be on the latest version of itunes

Ringtones transferred to your iPhone from your iTunes library on your computer or purchased from the iTunes store are available at Settings > Sounds.

Is this ringtone available under the Tones tab for your iPhone sync preferences with iTunes?

To make the Tones library folder visible in iTunes go to iTunes preferences and on the General tab under Show check Tones.  You should now see it under Library on the top left.  If you don't see the Tones tab of your iTune sync settings then you don't yet have a ringtone in your Tones library folder.