Planet Bike sells a Tailight Rack Bracket for your Superflash. Bracket cost is $5.00.
http://ecom1.planetbike.com/3108.html
You could also get one of their Rack Blinky 5's for $20 that included the bracket and the light but it's not a Superflash.
http://ecom1.planetbike.com/3018_1.html