Large file size? That makes it sound like you're trying to open a save state file or something, not the game's unmodified save data. The file I have successfully opened and edited is "PbrSaveData", size 3,584kb, located at "\Dolphin Emulator\Wii\title\00010000\52504245\data\GeniusPbr". I didn't have to do anything special to it to open it.
Once you do open it, though, you're going to have to change the TID/SID in the Block Data to make the game think that it's been hooked up to a DS before.