You let Bitpim do all the work right? You shouldn't have to add the ringtones to any folder, just have bitpim move the ringtones from your computer to your phone, and you should be set.
Make sure that the file isn't to big.
Also, make sure there are no foreign characters in the file name. Example's of those characters are: !, @, #, $, %, ^, &, *, (, ), -, +, =....and so on.
Make sure that the file name isn't over 12 characters in length, or the phone won't recongnize the ringtone.