I have configured exchange 2013 and I have added another domain using accepted domains. now I need to add a user on that new domain. how can I do that ? please show me the steps.

I'm a little confused by your question. When you add an 'Accepted Domain' your saying for you Org. "I want to accept email for this domain.". What this means is, your accepted domain needs to still be a valid, real domain. Is this domain a sub/ child domain of your root? If this domain you added does not exist, obviously, you cannot create a new mailbox for this domain. I think what you want would be if you had a child domain setup in your Org. and you created a mailbox within that domain.