2
Did you unzip the file? No need for "Tar" it is based on the "default" template. Skins are called themes here.
If not unzip the file and upload the folder called 0z_XTv2RC3 in the "themes" folder to your themes folder on the server.
Then go to system admin, preferences, general settings, and choose OZ as your default theme. Make sure you also have the "Default" template set as the default
Thats it
You will still see the existing theme you are in. The system thinks that is your default user theme you have chosen and remembers that you last used it. So in general settings you can also highlight the themes you want to be selectable if you decide to activate the "themes" block in blocks Admin. You make a theme selectable by going to "Selectable themes" in General settings. Highlight the themes you want to be selectable.
You can also go to Groups and make sure the Webmaster group is the only group that can select themes using this block if you wish. Handy for editing and trial and error, users still only see the default theme set up in preferences. This allows you to work on another theme, and users dont notice anything but the default theme set up