Depends on what you have in there now... don't mix types!
You should first find out why your system is leaking coolant.
WHEN YOUR CAR IS STONE COLD, top up to the cold max level on your expansion tank with the same fluid that is in there. Green is standard antifreeze, blue is BMW Coolant. If you see blue, be sure you aren't looking at your windshield washer reservoir
Watch/check for leaks once you get the car up to hot. Check for drips underneath, and look for tattletale drip marks and residue on the higher hoses, where the coolant may have steamed out and dried up.
Also, while you are under the hood, check your oil, and make sure its not like a chocolate shake. That would be a bad sign.