OK ... I figured it out by creating a record manually and exporting it to xls:
Interface 2: IP: 192.168.1.1, MAC : FD0BA8CE3491, TCP: null, UDP: null
Interface 3 : Blank
Interface 4: Blank
Is equivalent to the (single cell) entry where ":" is the delimiter for fields (IP/MAC/TCP/UDP) and "|" is the delimiter for records. Blank records are non-existent.
1:192.168.1.1:FD0BA8CE3491:22,80:|2:192.168.1.2:CFEA92867271::
DON'T use colons ":" in your MAC addresses. I generally found that for XLS/XML import/export it is easiest to stick to just alphanumerical characters, spaces and "." and and strip out all other characters from your data. Also, for null fields (free form - ie no restrictions) for it is safest to insert a string "null" or "unknown" as the XML import to AT seems to discard any empty fields causing your record columns to be misaligned when import.
Cheers!