Martin,
When you say it doesn't work, do you mean you get a Javascript error or that the extension applies the validation but then doesn't work?
If you get the Javascript error there may be a file or two missing from your installation so you will need to get a support call logged with WA so that they can correct it for you.
I find that the Unique Database Value validation works very well after getting over the Javascript error hurdle.
Cheers
Ian