go to Program Files\Eagle Dynamics\DCS World OpenBeta\Mods\aircraft\FA-18C\Input\FA-18C\joystick\default.lau and add these lines for 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')}}
after you do this a new category will be created in DCS for the f18 called specific switch abstraction, go to it and bind the toggle switch to it.
Hope this helps