Hi Cuacococo,
try this
Select
Hypervisor: QEMU/KVM
Check: Connect to remote host over SSH
Username: root
Hostname: <IP of you Unraid Server>
Check: Autoconnect
Then you may need to accept the certificate (type yes) and enter the password of your Unraid account.
Now you should see all VMs defined in your Unraid including Macinabox (if successful installed).
Hope this will help you.