you can take the Alilaguna Airport boat. It starts at Stazione Marittima and costs € 13 per person.
Look here for the timetable:
The Venice airport bus is cheaper: only three Euros. Look here:
For further information about the cruise terminal, click here:
Last but not least look at the port map:
You will find there a mark for Alilaguna.
If you need more infos, please ask.