When first getting the option to go to the dormitory you will have to walk up to the check-in desk which is to the left of the dormitory door. 

Once you have talked to Sophie at the check-in desk you can then walk to the dormitory and enter. This will complete the first day. 

After this you will have to finish some baby sitting jobs before you are told that you can go back to the dormitory to complete the next day.

You can only go into the dormitory when the game allows.