Thank you for contacting us.
The username and password for the sample website are admin/admin. They are set in the database. I am not sure why you are not able to log in. I have just installed the sample website on my local machine and it works fine.
You may find the following article useful : Resetting an Administrator Password
I hope this helps.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.