Make sure you qualify for a .us domain first:
http://www.enom.com/help/faq_dotus.asp
If I can help anyone, with one of my retail accounts (FREE to open) .us cost $8.45 to register. I am an ENOM reseller. You do get lotsa goodies incl. shopping cart, bulk features, free redirection, free email forwarding, and more goodies. No spam, just a service to fellow board members.