First you need to download substance bonus tools. Click here to download it. Make sure to choose correct version. After you download you can read the user guide that includes how to install the package. If you install correctly, you can see a new tab in Maya shelf.
Open hypershade and select the substance. Then click Convert the current Substance network to mentalRay ready network.
Then you can see a dialog box. Turn on gamma correction and click Create mentalRay shader network.