i don't understamd the op's question.
assuming they really mean download, like download from the web, then dropbox is an app that doesn't browse the web, so you'll have to download something into your browser first (icabmobile does this really well) and then move it from there to dropbox. there is no way to download directly into it. this is pretty much how you have to do everything on a regular computer as well. but, the extra step is no big deal.