Hi CosTan,
Sadly, the main game did not install properly - you seem to only be running the patch. This unfortunate event occurs due to a bug which is contained within the installation application.

To get BKB's mod up and running, uninstall the game and then re-install it.

After that, ensure that you have deleted the contents of "C:\Program Files\Total War\Medieval - Total War". This prevents any unlikely conflicts that may arise.

Next run the installer for version 2.0. Continue as normal until it queries where you wish to place the files. Order them to install into "C:\Program Files\Total War".

Click next, and allow the installer to work it's magic.

Once it has completed, open up M:TW to ensure that everything is in full working order. If there are any problems, please post them up so that we can attempt to help you again.

If everything is operating as it should, simply install the patch into the default folder. Afterwards, load up the game and, if the installation worked, you should be able to play away!

Hope this helps, sorry about the long response time, good luck, cheers!