Adding an alert sound to a Dash 3g

[LittleBreezes]

Hi all, I have a TMobile Dash 3G which I am trying to add an alert sound to. I have a short MP3 clip that I placed in the "My Ringtones" folder in main memory. I see that it shows up in the list of available ringtones but not in the list of available alert sounds. Where are the alert sounds stored, in what format and how can I add one of my own to the phone.
thank you very much
Frank