follow these steps
Download this file
N-Card Firmware 1.45 and Xmenu 2.53
After Downloading extract the files to any folder
1. Plug usb writer into your nds and hook it up till an empty usb slot on your pc.
2. HOLD A,B,L,R + Up on your nds to put the card into flash mode.
(Screen will say USB SAFE BLOCK)
3. Windows will pick up a fireline device, card is ready to be formatted.
4. Go to the folder were you extracted the download.
5. Double click NAND_ALL.BAT will will start the firmware flash
6. After a few seconds it will be done, switch of nds and switch back on holding Start & Select to format the card.
7. Windows will auto recognize the card again as a mass storage device.
8. Copy xmenu.dat from extracted folder to the root of your card. RESTART IN MENU MODE !
9. Card will create DSYSTEM folder at bootup. RESTART IN USB MODE !
10. Copy your roms, homebrew to the card...