I love seeing all these people from the 3ds section saying a DS flashcart labeled 3ds won't work in DSi.
As for OP (who already got it working but I'll post here in case someone needs the answer in the future):
Flashcarts will almost always have a website printed on them. Go to that website, find their downloads
section and download what goes on the SD card, which will normally be labeled as the kernel. The file you download will be a compressed file, probably either a .zip or .rar. Extract its contents using winRAR or 7zip, and then place the extracted contents to the microSD you want to use for your flashcart. Then you get your roms and place them on the SD card. I usually put them in a folder called "ROMs" or similar to separate them from homebrew apps.
I can't exactly tell you were to get roms, but I can say there is a certain Emulator Paradise
that I have used for years and I fully trust.