Download MeldaProduction universal installer, run it and make sure you select MSoundFactory to be installed.

MSoundFactory instruments may require samples and other resources, provided by the MSoundFactoryEssentials addon, to work properly.

Best way to install MSoundFactoryEssentials is MDownloader. Start it by clicking Menu and selecting Download and install products. Alternatively, you can download it manually and install using Menu / Install pack.

Install the actual MeldwayGrand instrument. Same as with the essentials addon, use MDownloader (preferred) or install manually.