What is the best way to meet up with the Liberty if you're flying into Rome?

Why not purchase airport transfers from Carnival?  Otherwise there is a train that takes about 90 minutes.

I must be confused--I thought Carnival provided transfers from the airport to the port?

Hi! I've been doing thorough research on this matter. If you don't mind handling your luggages, you can get a train from Rome Termini Train Station to Civitavecchia for about 8 to 10 euros per person. At the Civitavecchia station, it seems there is a free shuttle taking you to the port, which is about 1 km away. Taxis are also available for this short ride.

If you'd rather avoid the hassle with your luggages, several car with driver agencies offer the ride for about 120 to 170 euros for 2 or 3 passengers. Rates are higher if you are more passengers. They pick you up at your selected time in the lobby of your hotel and drive you directly to the dock. It may seems costy, but if I compare with the transfer rates I got if using Carnival services, they are competitive, reliable and trouble-free. I personally will select that option for my cruise on July 30.
Great cruise!

P.S. I forgot to give you some URL

www.romalimo.com (owned by an American)
www.airportconnection.it
www.romelimousines.com (owned by an American)

I think I do the private rate myself, its 160 for my husband from CC Dream.