should be straight forward.

if you go into "manage my booking" and click on the actual booking (getting that new all-colourful screen), you'll have a menu on the right hand side and one of the options should be "add EC number" - at least that's how I added a friend's number to a booking I had made for both of us a few weeks ago.

just had a look and it only gives me "add on business / venture club" but assume that's because all my current bookings have my EC number already. either that or they have indeed changed things.....