I love DSCraft, it's amazing what indie and homebrew developers are able to make for the Nintendo DS and Nintendo 3DS systems. You do need more than just an SD Card though, as the game won't load if you are just putting it on an SD card.
To play DScraft, you're going to need a flashcard like the R4 3DS found here: http://www.r43ds.org/products/R4-3DS-Dual-Core.html and then you have to download the freeware homebrew DSCraft game from the 3DS homebrew section here: http://www.r43ds.org/pages/3DS-Homebrew.html, where they have the link to download DSCarft,
It comes in two versions: the NitroFS edition and the Fat Only version.
This depends entirely on your card, but using the wrong version will simply throw up an error message - your flashcard and DS will be unharmed.
Download the NitroFS version, follow the next few steps, and if you encounter an error, come back to this step and download the Fat Only version.
Installing the game on your card couldn't be easier. Open the downloaded RAR file and find the folder 'dscraft' and the file 'DScraft.nds'.
Put both of these files onto the root directory of your flashcard or its included memory stick.
'Root', if you're unaware, simply refers to the main directory you see when you first open up your card, i.e. The first or top-most directory in the hierarchy
If all goes to plan, the game should now be playing. Choose 'Singleplayer', choose 'Testmap.map', and get started!
You move around with the D-pad, look by moving the stylus over the touchscreen, and create (or destroy) blocks by pressing the L shoulder button.
Double-tapping the screen makes you jump, and pressing select saves a BMP screenshot to your flashcard.
You can toggle between create and destroy, open your inventory of blocks, and save your map on the touchscreen.
To use different blocks, open the inventory and drag objects from the main area to the nine-panel dock at the bottom of the screen.
Hope that helps! This one game alone makes getting an R4 3DS card totally worth it. But as you'll see in the 3ds homebrew section, they have a lot of freeware games, emulators and applications as well!