To enter Jorvik City Mall, you need to have completed the quest where you help James get a delivery from the Jorvik City Mall. This quest is available at the beginning of the game, and you do not need to be a Star Rider to unlock the mall.
To get to the mall, you need to go by bus, which will be unlocked for you during this quest after being asked to buy a bus ticket. You can find the nearest bus stop right outside of Fort Pinta.