Yup, MegaFlashROM. I don't really understand the difference between the three carts, but it seems like you can just dump ROMs onto them right?
There are versions with extra memory, extra SD slots and SDD+/PSG, a sound expansion that lets many games sound better than the ones that just use the builtin Yamaha chip, like Metal Gear, the Gradius games, etc. Usually the SDD+/PSG-expansion gets detected on booting a game, just like the extra memory.
Yes, of course you can just dump roms on it and play them but some games require a bit of tweaking. The MFR supports cartridge (.rom) and floppy disk images (.dsk), many of which work right out of the box but not all of them. MSX computers were meant to be compatible across all the versions but some are a bit more tricky than others and have different memory mappers that may or may not be compatible to some modes of the MFR. As a workaround, there are lots of command line switches for the tool opfxsd which is used to load the roms.
The MegaFlashROM cart runs on MSX-DOS (64K memory machines) or MSX-DOS/Nextor (128k memory and up machines) which is a MS-DOS-compatible system. After installation of the MegaFlashROM cart, you will be presented with a graphical menu that you can use to browse your SD cart(s) and run games. This is done by simple scrolling to a game you want to play and pressing return. If a game does not run out of the box, you can exit the menu to command line and load it manually with the tool opfxsd.
You can also create a multiROM and load as many single roms into the memory of the cartridge as possible (up to 511), then you have to create a simple text file on your computer that contains the names of the rom files and optional command line options to load them, then transfer it to the SD cart and use opfxsd again to create a multiROM menu (command line switch /O (letter o)). As loading the multiROM games selection menu is done automatically on startup when you switch on your MSX computer with the MFR inserted, running games that way is quicker than booting into the file manager/graphical interface (it's called MultiMente, btw.).
It's highly recommended to read the manual of the cart and watch the tutorials on YouTube to get started.
This may sound a bit complicated to you and you will probably spend a few hours to get things going. Most of the time it's just a select-game-press-return affair so don't worry too much...
Speaking of disk images, there are two ways to use them with MFR. The easiest method is to select them and press return just like you would with a .rom file. The disk image file then gets loaded and the MFR cart emulates a floppy drive from which your computer boots the data. The simple method does not work with several disk images, tho, so you have to use the main command line tool opfxsd again. MFR also supports multi-disk games and writing to disk, this comes in handy if you want to play one of those longer adventure games like Ys II - Ancient Ys Vanished that support saving your progress.
IMO the MegaFlashROM cart is the best SD-based cart solution for classic computers ever made. I have a SD cart for my ZX Spectrum (DivMMC Future) that basically works very well but it's a very bland affair in comparison to MFR, it's hit-and-miss, games either work or they don't, and if they don't, there's little you can do about it. The MFR on the other hand gives you lots of options to run your games which makes things slightly more complicated but success rate is much higher in the end.
You might also want to check out this thread for some game recommendations:
MSX games and discussion thread.
If you have any questions or need help, just gimme a shout.