Did you already talked to Cid? If not first do so, he's in the pub in Poft. Then you need to go back to Alitar and talk to the King, give him the codeword Sunfire and he will brable something about a Godess Bell etc. Learn Godess Bell and go to the town where Josef is (I forgot the name of the town). From there you need to get the Hovercraft in the Mythrill-cave. And then you can go to the snowfields. To reach the snowfields go to Josef's hometown and then walk towards west. All the time till you reach see, then you go up and you will see an enterance to the snowfield. In the snowfield you need to go to the west and you will see the Snow Cavern after a while. Be sure to have good HP and magic, cause there is coming a very nasty boss.