Adding Heart Internet Domain Reseller to my WHMCS ?

Hi

I am wanting to offer my customers the chance to buy domain names from my site. I am going to use Heart Internet but cant see any info on how to do this. I am on 5.2.7 but when I click on configure on Heart Internet nothing happens.

I have created an account with Heart also. Do I need to do anything else with them ?

If there are some up to date instructions then this would be great ?

Thanks in advance

Martyn