To orbit around focused GameObject when running Unity on Linux mint you might need to change the Special key to move and resize windows setting first.
The keyboard shortcut to use orbit in Unity3D is Alt+Left mouse button. However that Alt key is by default set to be used for moving windows on Linux Mint 18 Cinnamon. This setting will collide with the Orbit setting.
To change this go to Preferences -> Windows -> Behavior and set Special key to move and resize windows to something other than Alt. I disabled this setting since I don’t use it very much.