XenServer: Add Network Interface Adapter (NIC) After Install.

xe pif-list

This will return the list of network connections currently seen, if you only see one (if adding a second card), or do not see the right one (eth#) for however many NIC's you have in the machine then proceed to scan for it.

xe host-list

xe pif-scan host-uuid=<host uuid>

Now check again.

xe pif-list

Unless there is a problem this should be all you need, now you should be able to see the NIC in XenCenter and XSConsole.