Click next to the ARM7 BIOS field and select your bios7.bin file.
While the BIOS files are critical for system startup, the dsi_nand.bin is the most complex part to acquire. It's a direct copy of the system's internal storage and includes the operating system, system apps (like the DSi Shop), and user data.
The BIOS is copyrighted by Nintendo. You cannot download it legally from a random website. You must dump it from your own DS/DS Lite/DSi/3DS.