You'll need to get it from HERE, and also make sure that you have a license key, which you can request when installing it...you'll also need to connect it to a specific e-mail address and choose a user name.
There are some extra features in the Pro version compared to the standard one... these used to cost hundreds of dollars to access, although I actually had a Pro version back in 2005-6 thanks to Dennis Reinhardt of Google... at the time I was working on the Earth: a Users Guide project, funded by the RGS-IBG, which became this blog.
If you have a previous version of Google Earth you'll need to delete or uninstall it first.
