If your Napoleonic Wars isn't detected by the installer, just follow this procedure to get it working.
- Press Windows + R
- Type regedit
- Head to HKCU - HKEY Current user
- Open the software folder
- Head to MountAndBladeWarbandKeys
- Right click on MoutnAndBladeWarbandKeys
- Click on New String Value
- Enter serial_key_nw as name
- Right click on the new string and hit "Modify". Now enter your serial key as value, which you can retrieve by right clicking on your Mount & Blade Warband on steam or from your buying receipt.