go to Program Files\Eagle Dynamics\DCS World OpenBeta\Mods\aircraft\FA-18C\Input\FA-18C\joystick\default.lau and add these lines for the barometric radar and masterarm
{ down = SMS_commands.MasterArmSw, up = SMS_commands.MasterArmSw, cockpit_device_id = devices.SMS, value_down = 1.0, value_up = 0.0, name = _('Master Arm Switch - ARM Else SAFE'), category = {_('Specific switch abstraction')}},
{ down = HUD_commands.HUD_AltitudeSw, up = HUD_commands.HUD_AltitudeSw, cockpit_device_id = devices.HUD, value_down = 1.0, value_up = 0.0, name = _('Radar alt Else Barometric'), category = {_('Specific switch abstraction')}},
This will create a new category in DCS, map the switches to those settings and you should be good to go.