Does the red LED on the key illuminate when the buttons are pressed? If no, then like edjack mentioned...have you changed the key's battery? (see text in red box in pic below)

Your model year probably requires a single 2016 battery:

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Q {BMW CCA Member #191509}
2006 E53 X5 4.4i (Sterling Gray) 12/05 mfg date <<~>> 97 740iL (Arctic Silver) 3/97 mfg date
(SOLD)
99 540iT (Orient Blue) <<~>> 95 525IT (Alpine White)
91 735iL (Schwarz Black) <<~>> 85 325e (Bronzit)