Open the caliper? Hmm. I've never done X5 brakes, but on my other cars (ford, Porsche, BMW, Acura) there is not allen key... just unbolt the caliper BRACKET from the front ..part number 3 in this dwg:
RealOEM.com BMW E70 X5 4.8i Front wheel brake-Brake pad sensor

Many people unbolt the caliper only to find the bracket must come off to change rotors...also, it may be easier to just do the bracket.

Hope this is of some use... Gotta think there is a DIY here too.

A