Once content was successfully activated on the Ubisoft Account you are currently playing the game with, you should be able to locate the content as follows:
Content of the Imperial Pack
Once you reached the 4th citizen tier, the Engineers, you will be able to build a Railway that connects the Oil Refinery with the Small Oil Harbour. The default locomotive will be replaced by the Imperial Train.
The Imperial Command Ship will replace the starting vessel.
You can access the 1602 Memorial via the decoration menu of the Farmers.
Content of the Season Pass
The company logos Promethean Fire and Eagle Crest can be found on the logo selection screen when you set up a new company.
Content of the Anarchist
The two new ornaments, need to be unlocked in the Ubisoft Club and can be accessed in the Decoration menu.
Content of Sunken Treasures
Content of Botanica
Content of The Passage