SECOND HAND ZYXEL NSA320S - HOW DO I SET IT UP?!

Hello all,
I recently purchased a "second hand" Zyxel NSA320S. I purchased it at a local auction house so had no direct contact with the seller.
I am trying to set it up - i have wiped the previous owners data from the hard drives and have done a "factory reset". It won't let me run the "initialization wizard" as it seems i may need the previous owners username and password? Is there a way to completely wipe it and set up my own username/password?
I have tried registering the product on the Zyxel website using the mac address and serial number on the box but it says they are not recognised!
Any help would be greatly appreciated!
Many thanks in advance!
Matt cole