I guess you are being hit by the Safari “open trusted files” syndrome.
Tell Safari to not open trusted files (Go to Safari preferences and uncheck that option), now you will get a .zip downloaded and untouched. As the Safari unpacker has a tendency to ruin the unpacking of installers that include zipfiles.
Also, be sure that you download to a place where you can write (the default ~/Downloads is a good spot) , but some third party apps can change that.
The zip then expands to a <installer_name>.app installer application when you unzip it with the built in Archiver, do not run any “third party” special zip unpackers as they will most likely go into the installer and unachive the installers internal zipfiles as well (often what happens) which ruins the installer.
Hope this helps, if yo still have problems, send me a pm.