That is the bad news. We walked it but it was a long walk. Probably about a mile and a half. Our ship (Carnival Freedom) docked at the furthest point in the port from the train station.
I don't know if you could talk a taxi driver into taking you to the train station but if you could I would suggest that. Look at this map
and the pictures before and after it. Our ship docked at the area marked "E12" the metro is just above that green square on the map. Its not hard to get to (just follow the edge of the port until you see the bridge in my picture) but it is a fairly long walk
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007