If Alueue's method fails to work, you'll have to do it manually. Here is an image from Red's post in The Big Help Thread:
As you can see, this is the Ogre.cfg file that is located in the main FeralHeart directory. Simply right-click to open with Notepad (unless you already have it specified to open these files in Notepad). Change the top
Render System=Direct3D9 Rendering Subsystem to
Render System=OpenGL Rendering Subsystem.
If THAT fails to work, see
this thread by Red. Just an extra step that may not be needed.