You can call from you cabin or go there in person. The staff there should be able to guide you to the best night to go based on Adventure Ocean. Some people like to go there to avoid formal nite or avoid a particular cuisine in the main dining room. The Host will have a dining schedule handy to help you decide. I don't recall if Adventure has a steakhouse or not (didn't when I sailed her) but if they do, I highly reccommend that as well.
You will have to book Portofinos as soon as you board the ship. You cannot book it before the cruise. And no....Adventure does not have a steak house.
2/13/09 Carnival Paradise
2/22/09 Freedom of the Seas
83/Azure Seas; 84/Song of America; 88/Stardancer; 92/Jubilee; 96/Viking Serenade; 97/Holiday; 99/Enchantment OT Seas; 00/Elation; 01,02,02,03/Ecstasy; 02/Voyager OT Seas; 03/Victory; 04/Adventure OT Seas; 05/Sapphire Princess 05,05,05,06,07/Monarch OT Seas; 06,07,08,08/Paradise; 06/Freedom OT Seas; 07 Pride; 07 Pearl; 08/Valor; 08/Serenade OT Seas; 08/Coral Princess