Please create a sub-folder naming 'IISP' or 'iisp' under the folder of '/modules/registrars/' in WHMCS, upload the enclosed plugin file to the sub-folder of 'IISP', find 'setup' > 'Products/Services' > 'Domain Registrars',  find 'IISP' in the dropdown list of 'Choose Registrar Module', add your username, password and Email address, then save the settings.
Select 'setup' > 'Products/Services' > 'Domains/TLDS', then 'Save Changes'.