@wildmay First, please make sure that there aren't any remnants of Dragon Valley in your game folder. Open the launcher and scroll through the list of installed content, and uninstall DV if you see it. Then look in Documents\Electronic Arts\The Sims 3\Installed Worlds, and delete the Dragon Valley file if it's still there.
Now try to download DV again. If you have trouble, try a different browser—Firefox and Internet Explorer work best with the store. You can download from your purchase history or directly from the DV product page.
If this doesn't work, go into Documents\EA\TS3 and move the entire Downloads folder to the desktop, open the launcher so a new folder is created, and try downloading again. If that doesn't work either, move the entire Sims 3 folder to the desktop, open the launcher, and try one more time. If DV does download this way, you can move the Sims3Pack into Downlaods inside your old game folder, trash the new game folder and put the old one back, and then install the world normally.