I had the same question re synching a key to a profile. They set it up for us at the PDC with one key but, when we arrived home, I tried everything I could think of to synch my key to my settings. Reading the manual didnt' help me (terminally dumb, I guess), so I called the nice lady who did the car intro at the PDC and she patiently explained it to me.
Manually selecting the desired profile via the iDrive did prove that we had 2 profiles configured. This is your first step. Make sure there are multiple profiles.
The next steps are easy and are related to locking the vehicle, just as Doug in NC stated. Select the profile you want to use via iDrive. Exit the vehicle and lock it using the key you want to synch. Don't use the touch pad on the door handle (if you have one); use the key to lock the car. It should now be synched to the last iDrive-selected profile. If you unlock or just open the door with that key in hand (or pocket), the seat positions and other options associated with that key and its profile should be set.
A good service guy should either not change the settings or save and return them to the customer's original. Can well imagine that a lot of them don't bother.
Dave @7500 feet on the plains in Colorado
2011 X3 35i - Blue Water Metallic