The Part 2 Assignment has to be downloaded from Sun Site, but I am not sure about the voucher bought from another party.
I have paid the amount through Sun web site, and they contacted through email with the information of how the assignment can be downloaded. The email contained URL to assignment database, the user id and a temporary password for me to log in. Once logged in, I had specific instructions on how to download the assignment
Since you bought voucher, I hope it is better to contact Sun using email address
who2contact@sun.com. They respond promptly
SCJP, SCWCD, SCBCD, SCDJWS, SCSNI, SCEA 5, PMP, Six Sigma Green Belt, Certified Scrum Master