Actually, I wondered also about the scenarii, but it was not about how to run them but
how to find them!
Scenarii are NOT directly accessible from the regular BTS/FFH menu. They won't appear under the menus "Play Scenario" or "Custom Game".
As the scenarii are linked (winning one unlocks the next one), they can not be loaded from the general interface as probably the routine for locking/unlocking would not be triggered
> For
FINDING the scenarii, you must create a regular game (PLAY NOW).
> When you are in the game screen, the shortcut is "
CTRL + E", or click on the new button that is on the top left of your screen (with the adviser's & Somnium buttons).
This will call the general internal scenario menu...
Now, for running them, as said before, for the moment it is a bit with magic and trickery and you need to try the various workarounds in this thread

But I guess the best is to follow Kael's workaround
(italics: my comments)
1. Start a custom game on a duel, erebus map. (with all default options, especially option boxes)
2. Save the game (using the interface, not fast save)
3. Exit FfH2. (I exit from BTS, back to desktop)
4. Reload FfH2.
5. Load your save game.
6. Then try to play a scenario.
It worked for me (Vista Ultimate-sp1, on a Dell laptop XPS M1730)