Hello there @Uneasedbow82! Welcome to the forums
I'm sorry to hear that you've been unable to access "Siege of Paris". Just to confirm, have you installed the content to your console? You'll need to install the expansion before you're able to access it, even if you own the season pass.
To find the content, you will need to go to "Manage game content" in the Xbox system. "Siege of Paris" should appear here to download.
If you're unable to find the content here, please don't hesitate to update us so we can investigate further.
Thanks!