From the web site:
Tickets must be purchased on usairways.com and travel must be completed by March 31, 2004 to receive bonus mile credit. Mileage bonus can only be posted to the member's Dividend Miles account for travel on flights operated by US Airways, the US Airways Express carriers and US Airways Shuttle for which the member has chosen to earn Dividend Miles. Please allow three weeks after travel is complete for miles to be posted....
However, I have to call about every two months or so because mine just quit posting. They always fix it, but it does become a pain to have chase them