Create a folder on your internal storage or SD card called "media". Inside that folder make a folder called "audio". Without quotes obviously lol. Inside that folder make a folder called "ringtones". Put all of your custom ringtones in there. I have mine in MP3 format and it all works fine.
You can also make folders named "notifications" and "alarms" inside the audio folder and add custom sounds to those as well. It's all picked up by my phone as soon as I add them.
I've been using this method since FroYo and it never fails. Also, I believe you can skip the "audio" folder step and put the ringtones folder directly into the media folder and it should work just the same.
Also, I would make the "r" in "ringtones" lowercase just in case...you never know when OSes will begin getting picky lmao.
Good luck.