Your newly installed VM will have a single English (US) keyboard layout. To add a polytonic Greek keyboard, first open System Settings (the far-right icon on your dock). Open “Keyboard”, and choose the “Layout” tab. Click the “+” sign at the lower left to add a new keyboard layout. For Greek be sure that you set the language to Greek, and the layout to polytonic, like this:
At the top right corner of your window, you should now see a menu item with a list of installed keyboard layouts, like this:
You can cycle through your installed keyboards with the keyboard shortcut
Repeat this to add any other keyboard layouts you would like to use.