Make a folder on the SD card - mp3. In that folder make another folder - ringtones. Put them in there.
If that doesn't work, try just a folder on the card named ringtones.
I've done it both ways on so many different versions of Android that I don't remember if both always work or one works on some versions and the other works on other versions.
Wherever it works, you can also make alarms and notifications folders, and put stuff in them.