With the OE hitch kit you need a hitch ball mount which fits into the 2" square receiver, drops the ball to a convenient height, and has a 2" (or similar) ball mounted on it. You need one with the correct size ball for the trailer you will be towing. I can't remember if the hitch mount came in the kit or I bought it separately when I bought my hitch kit. If you don't have the BMW one, any one will do, as long as the drop and reach (from pin to ball mount) are within the BMW spec, ie not too long or too extended.

The OE hitch kit also includes an electrical adaptor that plugs into the round socket under the flap, and converts that to a flat four pin connector commonly used in North America. If you don't still have that adaptor, U-Haul sell a version of it for a few bucks. You can see a picture of this adaptor in post 10 in this thread, right above the white trailer wiring module. It has two ears on it, is cylindrical on one end, and has a four pin flat connector on the other end.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue