The real aircraft is configured for Kilograms or Pounds and Feet or Meters in a device called the "Configuration and  Strapping Unit". To set these options on your simulated Challenger, go into the Airframe Manager, choose Edit Options, and look at the CSU tab. The setting is per-airframe, so you can have both metric and US customary airframes. This option is available in Career Mode - the airframe in Non-persistent mode is only available as it leaves the factory, in pounds and feet.