but says it doesnt pass authorisation and directs me to install InterLok extensions.
Then that is most likely why it isn't working.
I thought with demo didn't need authorisation for first 10 days?
It is an iLok plugin, which usually means to use it, you actually need an iLok, and you download a time-limited demo license to the iLok which lets you use the plugin.
I don't know about that particular pluginm but I expect that is how it works - the same with Syncrosoft-protected plugins - you still need the dongle to install the demo license on to use it.
Now, perhaps the Arturia has a variant of iLok which enables a temporary hard disk authorisation and so doesn't need a key - but you'll still need to install the Interlok drivers to enable this to work. The fact that you are being prompted means you have neither the Interlok drivers active on your system, and/or you don't have an iLok.
Does the demo come with an installation readme?