Many installations using a VM look something like the following:
Here is an example of a computer running the VM with 2 nics. One defined for the promiscuous mode (deep packet inspection for the Intrusion Detection Service) and one for communication to the NetWatcher cloud.
The virtual nic setup is usually where the issue occurs. Here is an example using VMware workstation. in this example you need to ensure that both nic's are bridged.
Bridged Mode : Your host computer (the Mac) will share its network connection with the virtual machines, they'll be sitting as if they were another computer on your network, everyone on the network will see them and be able to interact with them.
Both nics should also be connected, with the power on.. Choose the 'configure adapters' button and ensure you line up the virtual adapter with the correct physical adapter as seen in the next two figures:
You may also want to configure the sensor to have it's own static ip addresses (See here)
On the nic that you are using for permiscous mode you should also set it statically but leave it blank (by just hitting enter thru the ip settings) for that nic as getting an IP address on the Mirror may confuse the sensor.
then reboot the sensor and let it start up.