If you are Windows user, try this to confirm you have indeed changed the extension to m4r:
1. After creating the AAC version of the song you want to use as ringtone, right click on the AAC version.
2. Click "Show in Window Explorer". This will open the window where your AAC version is stored.
3. In the Explorer window, press Alt key on your keyboard. This will bring up the menu bar above your toolbar.
4. Click <Tools>, then select <Folder options...> to bring up the pop-up window.
5. Select <View> tab. in the Advanced Setting box, look for "Hide extensions for known file types". Uncheck it by clicking the box.
6. Now you should be able to see all the file extensions. Check that your AAC version that you created earlier has indeed m4r extension.
If you did not do this but simply rename your file with ".m4r", the actual file extension would have still been .m4a For example, if you right click your AAC version of the song LinkinPark_BurnItDown and rename it by typing ".m4r" at the end, the filename simply became LinkinPark_BurnItDown.m4r.m4a, so it would remain a m4a music file.
I also read numerous "how-to" articles and didn't get it to work, but I came across this one article that mentioned these steps, and that did it for me. Hope it works for you too.