So I just found out that I had Morrowind installed on my hdd. I haven't played it in ages, but I decided to give it a try. First a modded it a lot so it would look nice and then I started it up (using Morrowind Script Extender). Everything went fine, and I got to Caius Cosades. Then i thought that I should see if the expansion packs worked properly.

I joined the guild of fighters, and slept in one of their beds. When I woke up, a wild exclamation mark appeared! Several actually, one for each of the assassin's armor pieces. I figured that i perhaps didn't install the XP's in the right order, or that the bsa-files haden't been registered. So I checked in the archive section of Morrowind.ini, and changed

Archive 0=Bloodmoon.bsa
Archive 2=TR_Data.bsa
to
Archive 0=Tribunal.bsa
Archive 1=Bloodmoon.bsa
Archive 2=TR_Data.bsa

and started Morrowind. I loaded an erlier save and slept again, but now nothing appeared. Nothing. I changed back to
Archive 0=Bloodmoon.bsa
Archive 2=TR_Data.bsa
and did the same thing. Still no assassin.

I've tried travelling to Solstheim, thak works fine.

Does anyone know what I have done wrong? I would appreciate some help...

The installation order should be Morrowind then Tribunal then Bloodmoon. You could try reinstalling Tribunal then Bloodmoon. It may take a while for the assassins to appear as they attack at random times. The exclamation marks mean that the textures [ or meshes, I forget now which one] didn't load. Are you using any mods to change the armor? What does your warnings,txt say?

OK, I changed it to
Archive 0=Morrowind.bsa
Archive 1=Tribunal.bsa
Archive 2=Bloodmoon.bsa
Archive 3=TR_Data.bsa
I didn't know that the time of the attack was random, so I slept a few times. Voilà, now the assassin appeared, without exclamation marks. Thanks for your help!