Comment 9 for bug 336994

This was solved in the VirtualBox forums (http://forums.virtualbox.org/viewtopic.php?f=3&t=15512). Essentially what you have to do is edit the install script.

Step 1: run the regular export script with
     sudo ./VBoxLinuxAdditions-x86.run --target ~/tmp/
   where ~/tmp is any folder where you want to extract a bunch of files to.

Step 2: go to that folder (~/tmp/ in my case) and open the install.sh file. Go to line 415 and change
    15 1.5.99.* | 1.6 )
   to
    15 1.5.99.* | 1.6.0 )

Step 3: run install.sh
    sudo ./install.sh
  after this, just restart and everything should work properly.