Couple of pointers. Firstly you probably won't be able to enter the e-ticket number since there are some drop downs with the Cathay and Dragon prefixes hardcoded into them.

For the PNR, that has to be used with your name, and they can be a but funny about middle initials, so in Cathay terms my first name is CorporateW, as it were.

Also, first off get your BAEC number into Cathay. Then log off and leave it a few hours, then log in again and it may be you can select seats. I presume the link to the BAEC database is not entirely instant. Or indeed just ring up, they have staff in London who will quickly fix things over the phone. If you can get access to ExpertFlyer or some other tool to see what is available, so you know what to ask for, then it will only take a minute or two.