Hi Phill,
The converter you've bought converts an analogue signal to a digital signal. So if your speedo only takes an anlogue signal then it's gonna be pretty useless to you.
Have you definitely got an analogue speed sensor on the bike?
I've been tinkering with a Koso speedo that was fitted to my FZ600 (99) when I got it. Like you, I also purchased the same Koso analogue to digital converter, assuming that my Fazer has a analogue speed sensor. Upon checking, I found out that it doesn't. It has a digital one.
You'll see it on the left side of the front axel (pictured below).