How can you go to the new world in medieval 2 total war grand campaign

you can't. new world is there just for "decoration". you can play with new world in Kingdoms expansion set.
On the contrary, the new world is not just "decoration" in the game. You have to wait until they discover that the world is round which happens in approximately 1400. Once this happens and you are up to date on your port building, you can build a port that will allow you to build carracks. once you do this, you can explore west until you reach the new world. There are several factions to fight on the Cuban Island, Florida, Mexico and Brazil.