You will have 1 tile under your Games tab in the Uplay Client named Heroes of Might & Magic VI, but with the Shades of Darkness box art.
All content is accessed through the original game client.
Whether you owned Heroes of Might & Magic 6 before or not, you will still be downloading the full game. The differences are as follows:
If you owned Heroes of Might & Magic 6 prior to installing Shades of Darkness, you will have all core campaign content unlocked as usual, with the Shades of Darkness content added after activation.
If you did not own Heroes of Might and Magic 6 prior to Shades of Darkness, the core campaign will be locked and you will only have access to the Shades of Darkness content after activation.
To gain access to the Shades of Darkness content, you must make sure that you are patched correctly to Version 2.1. You can find your current game version in the bottom right-hand corner of the game's main menu. If you are at v1.8 and your Uplay Client will not automatically patch the game, uninstall and reinstall your game (Be sure to keep your profile & saves during the uninstall).
After the installation is done, launch the game and continue to patch it (this will require you to launch it several times until you have reached patch v1.8). You now need to download and install the v2.1 patch from the following links depending on which version of the game you own:
Digital version (English/French/Italian/German/Spanish)
Retail disc version (English/French/Italian/German/Spanish)
Install the patch and run the game. You can now find your Shades of Darkness content under the Unlock Content option from the Main Menu.
If you have any further issues or your problem persists create a support ticket. Please include what problem you have encountered, including your CD key and what version of the game you are currently running. Again, the version number can be found in the bottom right-hand corner of the game's main menu.