You have to wait until your psplus service runs.
I have psplus and i bought Burn Zombie Burn two months ago at amazon, when i tried redeeming it i got that that it was invalid but it worked when i used another one.
I liked the game, i loved the story but the spell animations just killed it for me and i just lost the patience as the game progressed since the fights were longer and longer because of it.
$13.99 on ebay with free shipping and he has 57
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220282275505&_trksid=p3907.m32&_trkparms=tab%3DWatching