Hmmm, Maybe i'm being really dumb here but how do i join?

To qualify for the promotion i need to join Diamond club first, in order to enter my membership number into the web page. To join Diamand club i need to

"Simply download the application form and forward it to us, together with two original boarding cards and corresponding ticket receipt from bmi for travel completed within three months of the date of your application."

So, as far as i can see... no boarding pass/tkt = no membership.

What am i missing?