You can log into the Spam Panel at the Domain level for your clients from the WHMCS Admin area. To do this, go to Setup > MarketConnect

Click Manage in the Spam Experts section

Type in the domain you are looking to manage or select it from the drop down list.

If the domain is not here then it has not been configured in your WHMCS system.

Once you have clicked the domain you should be taken to the domain login.antispamcloud.com and automatically logged into your Spam Experts account. 

The dashboard should look like the following screenshot. If it is blank this could mean your products have expired or are not yet provisioned.