codec for express scribe

I am currently trying to play a .dct file on my newly downloaded ExpressScribe application on my Mac OS X system. The program won't load the file and won't play. I have tried to convert the file to mp3 but this doesn't seem to work either.

An error message flashes up every time I try to load the audio file saying that my system may require an audio compression codec, but I do not know which codec I need to install or where to get it from.

Can anyone help?