As noted by esherflyer, do you have an avios.com account (avios.com is different from BAEC) and are you resident (or can supply an address) in a country where you are allowed to join avios.com? Your BAEC number is not an avios.com membership number (even when added to six other digits as suggested above)
http://www.flyertalk.com/forum/briti...avios-com.html