You need VBoxManage in your PATH. On OSX you can do this by adding /Applications/VirtualBox.app/Contents/MacOS to your PATH.
export PATH=/Applications/VirtualBox.app/Contents/MacOS:$PATH
require "virtbox"
vm = VirtBox::VM.new("vm-name")
exist = vm.exists?
vm_snaps = vm.snapshots
vm.revert_to_snapshot "snap-name"
vm.stop
vm.start