In order to earn Tier Points and Avios on US Airways you need to ensure your BAEC number is entered into the US Airways booking. This is how you can do it on bookings main directly with US Airways.
1) Do not create a US Airways Dividend Miles account!
2) Go to the main US Airways login -
http://www.usairways.com/
3) You should then make your booking via Book Travel, or you have an existing booking, go to Your Trip, enter the locator (booking reference, PNR) and the date of travel - yes this is different to other airlines!
4) In Passenger Information section click on Passenger Information, which is immediately below the heading. If there is no line then you will be precluded from doing it yourself.
5) In the Airline drop down box select British Airways
6) Enter your BAEC in the Frequent Flyer # box.
See below for an example.
You can also present your BAEC card at check-in, or in a US Airways lounge, however there is a greater risk that your Avios and Tier Points will not automatically post to your account. Therefore it is best to ensure your BAEC details are in the system 48 hours before departure. If you can't do it online, then it is worth a telephone call to US Airways.