Configuring ResellerClub (v 4.5)

Login to the ResellerClub control panel (the url will be in the email you received when you signed up with them) and then go to Settings > Personal Information > Primary Profile - the value you need to note is the Reseller ID.
Still in the ResellerClub control panel, go to Settings -> API and enter the IP address of the server where WHMCS is installed to authorize it for API access
Now, login to your WHMCS Administration Area
Go to Setup > Domain Registrars
Choose "Resellerclub" in the registrar dropdown menu and enter the Reseller ID noted above and password you set when you signed up.
Then click Save Changes

And that's it, WHMCS will now be able to communicate with your ResellerClub account to automate domain registration & management for your customers.

ResellerClub - WHMCS Documentation