To change or set your Blackberry STORM default 'Send Using' on the calendar and the messaging go to the main menu > option > advanced options > default services. There you can set the calendar and the messaging default services
I read this trade who sail that it cannot be set manually (How do I set a default 'Send Using' account on my Berry). Maybe it was true at the time but the thread was closed is there a way to link the old post to this one ?