virtualbox.org

I have a VM in Windows Server 2008, it works with a monitoring system, so I need an external access.
How can assign an external IP to the VM?

Virtualbox is a hardware simulator, and IP are a software matter - so you would change it into the guest OS, just like you would change it on a physical machine.
You'll need to set the VM Networking mode to Bridged for this to work, and use the public physical interface of your server as bridge.