1. Extract it to desktop. 2. Run the software named amnesia_tdd__1.2.1 and let it install. 3. After installation play the game. 4. Seed. (Optional) The file you have downloaded is a zip file, you have to unpack it before you run the installer or else the installer will not find the file containting all the data. You do this by right-clicking on the zip file and choosing "Extract All..." and follow the instructions.
Marus93