Hi,
Is this the first time you're trying to login to the ACS server? Are you using the default username of "ACSAdmin" with the password of "default"? You don't mention the version, but as per the ACS 5.2 User Guide.
The first time you log in to ACS 5.2, you are prompted for the predefined administrator username (ACSAdmin) and required to change the predefined password name (default). After you change the password, you can start configuring the system.
Regards