Updating a MacPro from Snow Leopard to Mavericks I have Snow Leopard installed on an old MacPro. I want to upgrade to Mavericks.  When I go to AppStore-->Mavericks and click on the download button nothing happens.  Do I need to install the intermediate releases (Lion and Mountain Lion - I think).  If so, what is the process?
If the Mac Pro is old enough, then it cannot be upgraded beyond Lion.  Check the listed requirements... My old Mac Pro 1,1 was in this category.  

See the chart at the bottom, "latest supported OS" HERE