The latest version of Call of Duty: Modern Warfare 3 is 1.9.463, and that's what you get when you download it from Steam. Our mod has been developed for an older version, 1.4.382, and thus after installing the game you will need to downgrade. Unfortunately, the developer does not make older versions of the game available, and we cannot host the files here because it would constitute copyright infringement. We must therefore resort to googling.
1) Open google.com
2) Google for "teknomw3 1.4.382 patch"
3) Pick a trustworthy looking download (I personally do not recommend the Tunngle one; it looks like it's infected).
4) After you've downloaded it, I recommend you check it with VirusTotal; if there are more than 1-2 detections, find something else.
5) Most of these downloads are packed in a RAR or a ZIP archive. To extract them, you will need an archiving program; I personally recommend 7-Zip. After you download and install it, unzipping the patch archive is simple; just right click on it, then go to the 7-Zip submenu and click Extract to ...
6) Now you simply extract the archive to the ROOT directory of your MW3 folder overwriting all files ( If it prompts you to ).