Normaly if you click on 'March Vue challenge' at the top of the vue forum you come to a page where you can find a link to the upload page. But the links there are still set to the previous challenge. So right now you can not upload yet.
Last month the upload page was only made a few days before the challenge ended. So no need to worry yet. Only thing is that you got to remember to check back in time to upload since we only get a few days to do so.