Hi my friend.
first of all sorry for my english, now i like to write something about transmission change fluid(because i have the same car): first you must open the plug 8mm bellow and dump the old fluid.After dump you put back the plug and you open the fill up plug (8mm i think )from the left side of the transmission(is close to where is connect the electrical loop to the transmission) and start to fill up your fluid.When is start to over flow the transmission you stop and put back the plug.Start the car and go back open the fill up plug and continue to fill up until to over flow again.at this point your are done(you must use between 6.8 and 7.5 lit). and for your info one year ago i use for 800km fluid (no bmw) from gas station normal atf (because i lost the fluid from my transmission) and was with out any problem.i believe is missing fluid from your transmission must do the operation from the beginning.
i hope to help you.best regards.