Remember that you can do Pset8 if you have a second computer (either desktop or laptop) running on either Windows or OSX.
Just use the appliance as a "server", the second computer (with GoogleEarth plugin) as a client, and ollow the instruction posted here to connect to the appliance using another computer.
Warning: this answer is related to version fall 2013 of the course
Yes, problem set 8 is optional. Quoting the professor's comment on a post on the course's Facebook group
In pset8, I am unable to get Google Earth plug-in working on my
computer then can i skip this problem ? If yes, then it is necessary
to inform CS50x team ?
This is why the pset says to use Chrome outside the appliance.
If you don’t have it installed on your computer already (outside of the appliance, not inside of the appliance), download and install the latest version of Chrome
Now head to the URL below using a browser on your own computer (outside of the CS50 Appliance), where w.x.y.z is your ...