I have a game controller but it is not used by the game.

The game supports only the Microsoft XBOX 360 controller or compatible. If you have another type of controller (DirectX compatible controller for example) the game will not use it.

If you have the Microsoft XBOX 360 or compatible controller but the game does not controls by gamepad please check if you have the correctly installed drivers for the controller.
You can download latest driver for your OS at Microsoft website with the link:
http://www.microsoft.com/hardware/en-us/p/xbox-360-controller-for-windows#support

If you still have no gamepad in the game please remove the %appdata%\Frogwares\the testament of sherlock holmes\setup.ini and then launch the game again.
To find this file please follow the steps:
1. Open any Windows Explorer window (The Computer window for example)
2. Type the %appdata%\frogwares into the address bar of this window and press Enter
3. In opened location find the folder named "the testament of sherlock holmes" and open it.
4. The setup.ini file must be there. Remove it and launch the game again

Also you can open this file with Notepad program, search there for "input_type" key and change its value from MOUSE to GAMEPAD. Save this file and try to launch the game.