Since you guys are getting the trans fail safe message, the easiest way to find out why you have the message is to have the car scan for trouble codes. It may not be the cheapest way, but it's a lot cheaper than driving around with a problem in the trans in which you can do additional damage to the trans.
It could be something as simple as low fluid due to a leak. A battery that is past it prime will cause the Trans fail safe message if it can't supply enough voltage for the electronic components, or a failing alternator.
If you want to DIY you'll need to get some scanning software to communicate with the BM Trouble U. I use Carsoft 6.5, BMW Scanner 1.4 and the app Torque.