Unable to subscribe to market data: Please assign an additional exchange to this instrument in the Instrument Manager
This message means you need to tell NinjaTrader what exchange this instrument trades on since it is not setup by default.
Please go into the Control Center->Tools->Instrument Manager.
Either edit the existing instrument in the list or add the instrument if it is not already added.
In the exchange drop down you will want to select the exchange that IB uses by check marking it as seen in the screenshot below.
You will now be able to access data for this stock from IB.