The core requires an SD-Card formatted for FAT16, and the first file in the root directory must be the MSX BIOS. However if it's inconvenient to use a separate card, then a virtual SD-Card also can be used in the form of a MSX.VHD file in the root directory of your main SD-Card. To prepare the storage, please refer to the KdL's site, especially OCM-Extra Pack, which has the BIOS and the SDCreate utility. Or you can find/ask for a .vhd file in Atari-Forum.
Go to mist-binaries. Copy the rbf file to the root of the SD card.