Hey all :)
It's a pain in the @$$ to use a keyboard to steer left and right in this game.
You'll find that out of the box, even if you re-configure the controls the bike only steers towards one direction.
You either have to use a controller or you must edit a configuration file to steer in both directions.

Navigate to the game input file:
C:\Users\<<your windows username>>\AppData\Local\MR4\Saved\Config\WindowsNoEditor\Input.ini

Use a simple text editor to edit the above file shown in bold (right click on it and select "Open with...").
I recommend using Notepad / Notepad++ / Atom to open the file. (Caution: DO not use Wordpad or Word or Libre to open it).

The file must be edited with the following three lines added near the bottom of the file:
AxisMappings=(AxisName="MoveRight",Key=Right,Scale=1.000000)
AxisMappings=(AxisName="MoveRight",Key=Gamepad_LeftX,Scale=1.000000)
AxisMappings=(AxisName="MoveRight",Key=Left,Scale=-1.000000)

These three lines must be added right above the "PitchForward" key bindings (do a word search inside the file, it's towards the end of it).
Now you should be able to use the arrow keys to properly steer left and right. Or any other keys you prefer, just replace the "Key=Right" and "Key=Left" with the proper key you wish to use.

Hope this was helpful. It's a pity the developers didn't implement proper key configuration in-game.
Note: This guide applies to Windows only.