Welcome to the forums, mickstafa. The Misterons Mix was only available as a free bonus download (for purchasers of all 3 Riverruns) until about 2009 or so. It disappeared after one of the many Shop redesigns, and is currently not legally obtainable.
You could try e-mailing
sales@underworldlive.com or
hello@underworldlive.com and see if maybe they could sell it to you separately somehow, or maybe even give it to you, since you bought all 3 Riverruns?