What are you trying to do ?
Is it an extension to the Main campaign or a stand-alone mod ?
1) Dialogs problem: I followed the larian video tutorial (basic dialog) but that didn't work....
Dialogs do require a few lines of Story Script to work. I am sure there are a lot of other things as well but let's take one thing at a time.
IF DialogStartRequested(_Char, _Player)
THEN StartDefaultDialog(_Char, _Player);
2) Camera problem: In the editor when I switch in game mode, the camera don't follow my characters, how to fix that ? If it's normal, can I expect the same problem in the real game ?
Camera works well in the game.
3) .pak problem: Can you explain me how to unpack the files (Which files ? Where I have to put them , Which unpacker ? For my understanding, why It's necessary to do that ?
It would be good to get an official respond from Larian to this question. However, when I made the UnderSiege I didn't unpack anything. But some things might have changed since I started.
Of course, you need to unpack some textures to unlock character portrait selection. The textures are located in Textures.pak in Public/Main/Assets/Textures/Icons/
The files can be unpacked using LSTools.