Notice: this information was written when Microthemer version 2 was current. On Dec 26th, 2014 version 3 was released with a completely new interface. Some of the information may be redundant, but as much of
the functionality remains the same in version 3 we haven't deleted this post.
Yes, that URL is incorrect. The actual zip file is in a secret folder. I’m about to upgrade to a different server as I’ve been having trouble with my current server recently. That could be the cause, although it could be a problem with Microthemer.
I will email you version 1.1.9 so that you can manually upgrade. And I will discuss ways to troubleshoot this permanently.