Open the FL Studio software and select the "Plugin" option located on the top left of the screen. Select the "Client" field, and choose the "Nuendo" option with the drop down arrow. Make sure the Nuendo software is installed correctly if it's not under the list of options.
Click the "Multi Outputs" button if you plan to have multiple outputs within the Nuendo software. Configure the MIDI connections by selecting the "MIDI Options" tab from the ReWire window if you're using MIDI.
Click the "Show Panel" tab to launch the Nuendo software. Open the software manually if it does not launch by clicking "Show Panel." The software will be Rewired once Nuendo is launched.