Active PDF Stamp feature - not working

I've enabled the Active PDF Stamp feature
Bypass paypal order form - No (so fields have been filled)
Tested by;
purchased a pdf, downloaded it, checked it and no stamp on any page.
What am I missing here? Is there something else I'm supposed to be doing?

I want to have each purchased document to have a unique ID stamp.

Thank you
Joel

Hi

To use the pdfstamp feature, you need to install pdftk library . Please look at the attached document for more detail information .

Thanks,
Tuan