Are you placing that link into Platform? You need to place the automatic download link into platform. This can only be obtained by placing the .zip file in the "Public" folder on your drop box. One you have that, you will get a "Public Key" by right clicking I believe. (IE:https://dl.dropbox.com/u/67646975/mods.zip). This is the only way that Platform will download your pack.
Edit: Otherwise, from what I can tell, your modpack looks to be correctly setup.