1. Open STK software and create a new project.
2. Select the "Add Map" option in the "Map" menu, and select the map containing China's national boundaries.
3. Right-click the map, select the "Add Object" option, and then select "Country".
4. In the country dialog box, select China and set its properties.
5. Drag the mouse on the map to draw the boundary of China.
6. Confirm and save your changes.
These steps should help you mark China's national boundaries in STK. If you need more help, you can refer to the official documentation of STK software or ask STK technical support team for help.