Not sure about the one you used but there are several offline archive decoders available.
Here are some for you to check out..
Yammy - Yahoo Messenger Archives Decoder
IntellectualHeaven - Products - YahDecode - Yahoo Message Archive Decoder
Yahoo Messenger Archive Decoder Details Page
Yahoo Message Archive Decoder: Read conversations, chat, passwords, etc of anyone without logging in! Decodes dat to html files! - Ikitek Software
Theses are just a few of the many that are out there, hopefully they help you.