What error does it give? My path reads:
"C:\Program Files (x86)\Steam\steamapps\common\amnesia the dark descent\Amnesia.exe" <space> "tetris_TDD\tetris_main_init.cfg"
With the mod (the cfg file) in the following directory:
C:\Program Files (x86)\Steam\steamapps\common\amnesia the dark descent\tetris_TDD
Edit:
It seems your folders are set up correctly from the information provided. The solution may be that you need a space before the part you add to the target (otherwise the path won't be valid). Have you tried this?