If you aren't logging in as root, the account you are using will need sudo access to enable proper scanning. (this can be checked by logging in and running "sudo su -" . You'll asked for the login-accounts password. If that works, you're set.
You then check the sudo box at the bottom, and enter the login account's credentials.
Shawn313, the above information all looks correct, but something to be aware of is that MVM will only authenticate to a system if it is running a check that requires it. Meaning if you are just running a discovery scan, or a scan with only checks that don't require authentication to the target device, then no authentication will be recorded in the reports. So if you are testing your authentication be sure to select checks that require it. For CentOS checks, just go into the vulnerability section and under shell there is a CentOS sub group. Those checks will require authentication to the CentOS device.
Also, most UNIX/Linux checks within MVM do not require sudo or root level access, though there are a few that do for certain UNIX/Linux OS types.
Finally, not sure where you are verifying your authentication status, but in the HTML report you need to look in the UNIX Host Assessment --> UNIX Access section under the "Report Pages", or if using the CVS report you can look in the authentication_hosts.csv file. You should see your access level under the protocol you used to authenticate to the device (SSHv1, SSHv2, Telnet, etc)