Oh dear. You've done the whole course, you've tackled assignments yourself, asked the tutor questions about what you don't understand, this is D-day and you've no idea at all what to do with the assgnment?
If your tutor is that bad then don't worry, everyone will fail and it will be the tutor's problem, not yours.
OTOH, if you've no idea because you've been getting other people to do your work for you, e.g. by posting assignments on internet forums and handing the answers in without working them yourself and figuring out why you couldn't do it yourself, then a fail on this course is not an unreasonable expectation. A programming course is intended to teach you how to program, not how to get others to do tasks for you; for that, you should be on a Management course.
But I'm not completely without mercy. It looks like a simple enough task. Complicated explanation, but very well documented code and by the looks of it you already have 75% of what you need to complete the task, basically you just have to reverse the code you've been given so it converts D to B instead of B to D.
Where are you stuck? Could you outline in English basically what DtoB() will do? Do you know how to convert decimal to binary manually, on paper? (If you haven't then you have no chance of completing the assignment; you can't program a task you can't do yourself.)
Do you think you might need to call BaseGen()?
Do you think you might need to loop over something, and if so, what?
If you're keeping "string Bin", how do you think you might need to modify it within the loop?