It is good to get a second opinion although Forums are only good estimated guesses that the problem that you have
Do you self a favor, spend the Diagnostic fee of 120-140$ at a BMW dealership and have them explain to you what the actual problem is with a car. ( unless the shop you went to is a BMW Dealer and not a random "Johnny's Import Repair" shop)
When it comes to transmission issues it is important to have indepth factory diagnostic scan tool figure out the problem. There is a chance that you could have an internal problem with the transmission which most shops will not deal with, it could be something simple as a broken wire harness or a fluid leak to the point of where you have leaked out so much fluid the transmission has a difficult time engaging due to lack of hydraulic pressure.