It is appearing within WACUP (Preferences > Plug-ins > General Purpose) but clicking on it then the 'Configure' button is not allowing me to configure it (no dialogue appear) - which I presume I would need to be able to log in to the service.
The config button being available to click just means the plug-in has reported there's a config method but there's nothing to indicate that it's not a dummy one (i.e. wouldn't do anything). iirc the last.fm provided plug-in didn't have a working config as it relied on their desktop client to handle it. The even older standalone last.fm plug-in did have a working config but iirc that won't show / crashes on modern versions of Windows since it was built in a manner that conflicts with security changes that have been applied to the OS since that time.