The case should have a specific template filled out:

Client Name:
HEP Shortname: 
Date to enable:

Once you get the case, change the Account and Contact to the client listed in the case, as it will arrive from HEP as an "email to case" case with none of that information filled out.  

To enable HEP for the client, go to the Site Settings (Setup > Site Options) click on GO then search for "DON_HEP_CUSTOMER_CODE".  

Enter the HEP shortname exactly as it is in the template and save.

The article explaining how to use HEP is here and you can send that to the client once the setting has been updated.