WHMCS Provisioning

I am having a real problem with getting this plugin to work with my whmcs install. Every time I go to add a site through whmcs it comes up with the same error stating the plugin is not active on the wordpress install. The plugin is active. Really struggling to find where in the setup in both my whmcs and wordpress I have not set things up correctly, and after spending the best part 0f 5 hours trying to determine where I have gone wrong could do with some help.