Hello All,
I have a nagios server with an external ip address , and then I have a server with a public ip with a couple virtual machines with private ips..I want to get nagios to monitor the VMS with their private ip addresses.

I cant find a guide on the internet, can anyone help? Is it possible?

Elegant way #1 would be to create another VM as a Nagios server for the private environment, use that to do the "internal" monitoring and then pass the results through to your external Nagios server using the nsca sender/acceptor.

There are a few tutorials around for this, Google Nagios NSCA or Nagios Distributed Monitoring for other clues.

Elegant way #2 create a VPN between your Nagios server and your private network and use Nagios "normally"

I'm sure there are other but #1 works quite nicely. We've an "office" instance of Nagios that accepts results from three other instances out at data centers and provides a central view.