MDrummer comes with a huge library of samples, multisamples, rhythms, drumsets, loops and other resources. These are distributed via the MDrummer packs. The pack MDrummerEssentials is mandatory and is needed for MDrummer to work properly.
Best way to install MDrummer Packs is MDownloader. Start it by clicking Menu and selecting Download and install products. Alternatively, you can download them manually and install using Menu / Install pack.