First of all, I would suggest getting a clean ROM and patching it with the DS-Scene Rom Tool (unless the ROM you got was patched with the rom tool, of course).

Anyhow, have a look around for an already existing save file (can't upload any myself because I lost it) and put that on your card. Play the game for a bit, turn your DS off and on again, and it should still be there. At least that's how I managed to fix it on my DSTT back in the day.