By default, it is impossible to access this area, because of the locked door. You can be granted access after you talk to Edward Deegan in Bunker Hill, where you start quest for the Cabots Special Delivery.
Another way in is to use the intercom and use the "bottom" dialogue option - Let me In. The person on the other side will lose their temper and let you in. Top floors of the building are inaccessible, by default, up until you complete all three quests that you receive from Jack Cabot. Items in the location often replenish.
In the basement, you can find Fat Man and a mininuke,
In Jack's lab there is pistol Zeta Gun on the table,
In the bedroom, at the top floor (which is inaccessible at first) there is an issue of Massachusetts Surgical Journal.