Thief 2 - T2X: Shadows of the Metal Age v1.1 addon

First of all make sure you're playing with T2X version 1.1. Check if the t2x folder has a file named 'version_1_1_readme.txt'. If it doesn't download v1.1 from a alternate link here.

If you're dealing with version 1.1:
You don't need to use t2x.exe. It was just supplied for backwards compatibility with v1.0. Just make a shortcut to thief2.exe and set its 'Start in:' folder to the t2x folder. And use that shortcut to start T2X.

• If that doesn't fix anything:
  What happens if you try to start T2X?
  
  Do you get to see the opening screen?

• If not but you can hear the opening screen sounds (waves and seagulls) you're probably dealing with a broken darkinst.cfg in the t2x folder. Easiest way to fix this is to open it up (using notepad or some other plain text editor) and change its contents to
  Code:

  cd_path d:\
  install_path ..\
  language english
  resname_base +..\res
  load_path 
  script_module_path +..
  movie_path movies

  Copy the cd_path value from the darkinst.cfg in the thief2 folder. If that value is a relative path (ie not a drive letter) make sure it points to the same folder. Most likely "..\" will do in that case.
  
  
• If changing the darkinst.cfg doesn't change anything chances are that some file did not get copied properly. I'd advise you to redownload and install. Be sure to copy ddfix.ini again.

K-Lite Code Pack issue:

Whenever you watch a video with WMP and an external program, ffdshow(A decoder packaged with the K-Lite Codec Pack) loads - and every time I closed WMP after watching one of the vids, it'd shut itself off. So I did a bit of puttering around with the ffdshow video decoder, and realized that it was set as a decoder for the VP3 codec that Shadows of the Metal Ages uses (just check what codecs its using and untick vp3). Turning that option off made the video work fine in WMP still, and, after trying 'em in the game, they run fine too.