You have noticed that the Credential Manager Plugin Agent doesn't appear listed as installed when you know it has been actually installed on your client machines.
Your "Install" policy (under Settings>All Settings>Monitoring and Alerting>Credential Settings>Credential Manager x64 Package Install) is enabled
ITMS 8.x, 8.7.x
This is working as designed. The install applies required registry keys. They are found at:
HKLM\Software\Altiris\CredentialManager\Keys
HKLM\Software\Altiris\eXpress\Altiris_CredentialManager_Agent\version
If it is suspected the agent wasn't properly installed, check these two locations to confirm the install executed successfully.
This install will not register a plug-in shown in the Symantec Management Agent's UI or in the agents table in the database.