If you have a brand new mac, the default is to download to a folder called Downloads which is sandboxed i.e... designed so that if you are downloading something bad such as a virus, the file can't execute outside that Download folder. You can change the location you download to or change what the Download folder does by going to the Security and Privacy tab and changing what the Download folder allows but read about this first so that you understand the security implications.  I would recommend that you get one of the many installer applications for Poser since you don't want to overwrite the runtime of your Poser application with the runtime of you new download.