Thank you for using our services.
The requested username and password is for the windows credentials on your computer. Try using those to log in. If you want to disable this, you should enable Anonymous authentication on your IIS server through the IIS Managment Console. Take a look at the attached image for reference on authentication settings in IIS.
I hope that this will solve your problem. If you have more questions regarding Sitefinity, please feel free to contact us.
All the best,
the Telerik team