Hi I am trying to start the designer through the URL "

Yes this is new installation and it is first time i have installed it on the machine. But when i try to start the designer it ask me for credential but i dont have any idea what to provide there. Also rest all engines status is okay.

If you have never been able to access the environment and nobody has been prompted to change the default password then the username is (case sensitive): Administrator
and the password is (case sensitive): admin

Anuj, you mentioned you were being prompted for the "XDB username and password". Is this in a popup when trying to access the Appian login page or are you actually getting to the Appian login page and entering credential like at forum.appian.com?

If you are seeing a popup for XDB credentials then that is not Appian prompting for the credentials. Check to make sure nothing else is running on port 8080. If Oracle is installed on the box I'll bet that is what is prompting you for the credentials.