1/ Go to Swiss.com
2/ Book flight
3/ On check out page select Diamond Club as your FFP and enter your DC number
4/ Fly
5/ Miles will post to your DC account
Where is the hassle? Its what everyone has to do when booking a flight.
Or are you trying to redeem your miles for a free ticket? Then you have to call bmi diamond club. If you want to book a revenue ticket then you always have to go to the operating carrier / travel agent to book, unless we are talking about codeshares but that is another story.