When I put the cart inthe gameboy, small startup sound, black line flashes on the screen, the power led is on but then dims, then nothing...
The gameboy is working with a game cartridge, any idea what's missing?
You're right that you shouldn't need to flash a sav file. Here are some things to try:
Put in fresh batteries
Test your lsdj.gb on an emulator to make sure it isn't corrupt
Clean your cartridge contacts (and the cartridge slot too, just in case)
Try another ROM on the cart
Do you see the Nintendo logo as the EMS cart starts to load, or do you see the "No Cart" black rectangle? If you got your EMS cart from Kitsch-Bent, it will probably have come with a small demo-ROM pre-flashed? If so, did this work correctly before you tried flashing LSDJ?