Are you attempting to locate the file on Rockbox itself or your computer?
You're correct about the location - it'll be in the root (the very beginning of the filesystem, in terms of folder location) of the device, with the filename ".scrobbler.log".
If you're trying to find it on Rockbox, check to make sure that the File Browser is set to show All Files, not just Supported.
If you're looking with a Linux computer, check to make sure your file browser shows hidden files.
If you're using Windows and Windows Explorer and you don't see it ... the file probably doesn't exist.
If you're on a Mac, I can't help you.
