Hi,
Yes it's all change for WorldPay in the latest version, but having gone through the hoops below, I am now working fine.
You need to get some new Keys....which means a new account with WorldPay, which is strange because I too already had an account with WorldPay for my OC 1.5 and when I called them they said there was nothing they could do.
I don't know if WorldPay have 2 different business's, one through a company called Streamline but the solution is to go here and set up a brand new account:
https://online.worldpay.com