Are you driving to the port from the airport?
If not-taxi fare is a fixed rate of $24 for up to 4 passengers, that's the easiest way to get there- takes about 15 minutes.
Here are directions-
We did the same last summer, spent 1 week in orlando then drove to miami. If you look on RCI or port of miami website they have the port information. Use goole maps or the port of miami website as you can get the address information. As far as I can remember its just off the I95, sign posted and quite easy to find. The area is made up of grids so if you take the wrong turning its easy to get back on the right track. I printed out a map from google which helped.