OK so do you want Mach to be native Inch or Metric? Will you be setting your motor tuning as Step per Inch or Step per mm? If you are wanting to set up the motors as Step per mm then do as Ed suggested. If however you want to keep the native units as inches but just run metric code you can do several things, first you ca have G21 at the start of your code or second you can go to config menu then General config, you will then see a box for Startup Modals, type G21 in that and Mach will always start in metric.
I think its best to have your machine set up in the native units you will use but its up to you.
Hood