Ajay,
For help adding a device into Zenoss look here: http://wiki.zenoss.org/Add_a_Device
To get the IP from the VM, login to it and type: ifconfig
If you have issues connecting to the VM, try changing the network from NAT to Bridge.
I would also recommend that you install and configure SNMP on the VM (found this link from google: http://andrewpakpahan.blogspot.com/2012/09/how-to-enable-snmp-monitoring-on-ubuntu.html)
Hydruid