Thought I'd just post a quick bit of info for those looking for Worldpay on Zen 1.5.x

I have just installed the latest module available from the add-ons section and all appears to be working properly. You can pay and get returned to the store after payment for the reference numbers though.

The worldpay option appears in the modules=>Payment section in admin, but not directly under the list of modules. Other than this it seems fine.