It seems to be missing from the joystick input file. You could paste this into the DCS World\Mods\aircraft\SA342\Input\joystick\default.lua file.
Alternatively replace the whole default.lua with the attached one.
You'll have to do this every time you patch or repair your game until it's added by Polychop. And make sure to back up your original file first.
{down = device_commands.Button_7, cockpit_device_id = devices.HEAD_WRAPPER, value_down = 0.0, name = _('SA342 Set Gunner Seat'), category = _('View Cockpit')}, SA342_default_joystick.rar