1. Make sure you're using External NIC for both Public and Private NIC (virtual switch)
2. When using Windows Server 2008 RTM, apply this hotfix KB953828
3. If the VM guest is using Windows Server 2008 with SP2, no need to apply the hotfix but reinstall the Integration Services setup and reboot the VM. Double check the public NIC. If problem, manually configure the ip address of your Public NIC and Cluster IP Address. Just add additional ip address. After you have done that, you can open the NLB console and you will see that your cluster has sucessfully converged.
Please refer to my below snapshot for the error when creating NLB cluster and adding new node.
Figure: Error when creating NLB cluster
Figure: When adding additional node
4. For R2, manually set the MAC address of the interface you’ve bound NLB to on each server in the virtual machine Hyper-V settings to the MAC address assigned by the NLB service to the network interface within the guest.
Command of the day:- use WLBS Query. More info:- http://support.microsoft.com/kb/242242
Why you need to configure like this?, please refer to John Blog, http://blogs.technet.com/jhoward/archive/2009/05/21/new-in-hyper-v-windows-server-2008-r2-part-2-mac-spoofing.aspx