Are you using the "Check for Updates" utility from inside Mixcraft? Alternatively, you can download any build straight from the Acoustica website:https://www.acoustica.com/mixcraft/download.php
When I experience DL problems, first I clear the browser cache and history, restart the browser then try again. If that fails, I try a different browser - say, instead of Firefox, use Google Chrome. One of those usually does the trick.