Try saving the attachment to your desktop, and then open the saved file from there. You can also drag the attachment from your desktop into your eShuttle Transcribe window to load it.
Even if Windows itself properly knows to open .dict attachments in eShuttle Transcribe, some email software or web browsers will not recognize the .dict extension. Most email programs have the ability to manually change file associations. Please check the Help files for your email software for more information.