How do I write the .ds.gba onto my flash cart?
.ds.gba files are written to a GBA flash cart in the same way as GBA homebrew is written. You must turn off any menus, as the menus are written for GBA and will not run on NDS. Linux users could try flinker: it works with Flash2Advance carts (not ultra). Perhaps using a CompactFlash is a better choice for Linux users. Flash2Advance page: http://www.gameboy-advance.net/fal_soft/gba_flash_2_advance_linux.