This is more of a memo for me than anything else.  I just recently downloaded JoliCloud to give it a spin before I decided what OS to run on my new netbook.  I am very impressed with JoliCloud and love its look and feel.  Here are the instructions I have found in order to make VirtualBox additions work:

Jolicloud doesn't come with everything you need to build the extra kernel extensions, so you'll need to install a couple of things. First you need to mount the Guest Additions ISO (click the Devices menu, the Install Guest Additions). Once you have it mounted open up a new Terminal window and type the following:

sudo apt-get update

sudo apt-get install make linux-headers-$(uname -r)

cd /media/cdrom0

sudo ./

Hope this helps out