You MUST mount the C drive before DOSBox will recognize it. You can mount to whichever directory you'd like. For example, if you want to mount your root C directory, type "mount c c-colon-backslash", where colon is the colon key : and backslash is the slash \ (I've typed them out here to avoid putting in an emoticon...in this case, the one that looks like :\ .

If you want to mount directly to your Starflight directory, assuming it's C: \STARFLT, type "mount c c: \starflt". Just don't leave a space between the colon and the backslash.