WHMCS Integration plugin

I have installed and can use Wordpress in the main directory of my site (www.mydomain.co.uk) and I have installed and can use WHMCS in another folder (www.mydomain.co.uk/whmcs). When I try to set them up in the WHMCS Integration settings in Wordpress, I keep getting the "WHMCS Integration: couldn't connect to host" error. Can you provide me with some help?