Steven,
I downloaded a fresh copy of com_wordpress_2.6.5.3.zip using Firefox to my desktop. For fun, I tried to install it and had the same result. I uncompressed the zip file and can see all of the constituent files and subfolders. I then compressed these files back into com_wordpress_2.6.5.3.zip and attempted to install, with the same result.
Am I missing something? Perhaps the term, "package", is something other?