Didn't you set a root password? (I've seen this type of issue on Fedora forums--apparently, people miss it.)
If so, that is, if you don't remember setting a root password, there's the possibility that at present, it's a blank password. (Though, with CentOS 6, that seems a bit odd, as I know if you choose an easy password, you get a warning that it's a weak password.)
Anyway, to see if that's the case, use root as the login name and try a blank password.
If that doesn't work, then maybe the install skipped that step, or
In that case, you can boot into singleuser mode (by, when you hit the grub menu, hit e, as in edit, use the arrow key to get to the kernel line, the long line that ends with rhgb quiet, and just put a 1 at the end--that's the numeral one, not a lower case L.)
Then, when it boots, you'll be presented with a login prompt. At that point, you can set a root password. (At the # prompt, just type passwd and it will ask for a password). When done with that, type exit, and it will boot into the way it usually does, at which point, in theory, you can log in as root with the password you just created.
Note, in case you're not familiar with this, that it's considered poor security to log in as root, and you should, upon first log in, create a user account.