You can activate the mental ray renderer for the entire scene, or for specific passes.
The Render Manager > Scene Render Options
From the Render toolbar, choose Render > Render > Scene Options
On the Scene Globals tab, set the Scene Renderer to mental ray.
The Render Manager > (select a pass)
Set the current pass and choose Render > Render > Pass Options or Pass > Edit > Edit Current Pass from the Render toolbar.
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