By default, LonMaker and LNS use the computer's regional settings to determine whether to use SI or US unit.
If you want to change to measurement system being used, try changing the regional settings in the Region and Language from the Control Panel. Make sure the language is set to a European or American language depending on your region.
Also, you might need to change the system locale from the Administrative tab.
If you don't wish to change the regional settings to a different country, you can try to select Additional Settings in the Formats tab, and change the Measurement System field only.