Are you trying to use the Captive Portal page on the controller or CPPM? It sounds like you are trying to get them to use the CPPM one; but what is they are seeing? Check the URL defined for the Login Page in the captive portal authentication profile on the controller for this user.
Configuration --> Authentication --> L3 Authentication --> Captive Portal Authentication Profile --> Your Profile Name
Login Page = ???????? Does it say /auth/index.html or does it point to your CPPM box?