The original ignition switches have 3 positions (plus "lock" if it locks the steering stem as well): off, on, park.
Having an "ACC" position identifies yours as an automotive replacement switch. The accessory position is useless on a motorcycle, on a car it's for listening to the radio with the engine off.
The connector issue is weird, I haven't seen a replacement automotive switch that would do that.
Anyway, I believe you need to switch the battery power (red) wire to the black wire for ignition to work. For the tail light, you need to also connect the brown wire to battery power. This will have the TAIL fuse doing nothing... but I don't see a way around that.
SO: red on the BAT terminal, nothing on the ACC terminal, and black and brown on the IGN terminal. Bown/white wire should be clipped and the end taped, it is be +12V power from the TAIL fuse.