That's because you don't change them using ACE for the L-1011. Go to the Fuel and Payload Manager to use KG/LB:

Menu > Add-Ons > CS 1011 Captain II > Fuel and Payload Manager

While you can use either KG, or LB, weights when using the Fuel and Payload Manager, I'm not sure how that affects the display in the INS. I assume that setting the flight sim to Metric helps you to see the metric values on the INS, and other instruments.