Hi Marco,
The Cisco NAC Appliance has to be installed on Cisco NAC hardware.
It's not working nor it's supported on VMware.
You can find the list of the supported hardware on this document:
http://www.cisco.com/en/US/docs/security/nac/appliance/support_guide/srvrhw.html
If you need/want to run a NAC solution on VMware, you may have a look at the new Cisco ISE (Identity Services Engine): http://www.cisco.com/en/US/products/ps11640/index.html
The details about installing Cisco ISE on VMware are covered here:
http://www.cisco.com/en/US/docs/security/ise/1.0/install_guide/ise10_vmware.html
I hope this helps.
Regards,
Federico
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