Hi mtwfan
This is a bug with the installers file path - you just need to do a little bit of manual overwriting yourself to get it to work.

If I remember rightly, to start with you will need to open up the 2.0 installer.

Follow the on-screen instructions till you get to point where it requests details of where you want to install it to. Give it the file path "[hard drive letter]:\Program Files". Typically, this will be "C:\Program Files".

Once the mod has finished installing itself. Navigate to where you "installed" it to ("[hard drive letter]:\Program Files" typically "C:\Program Files").

Find a folder labeled "Medieval - Total War" and rename it "Medieval - Total War - Gold Edition".

After that, locate a folder named "The Creative Assembly". Drag and drop the "Medieval - Total War - Gold Edition" folder into it. If things are going as they should the computer will request permission to overwrite files. Simply select "Yes to all".

Once the process is complete, you should, if everything went to plan, have a working version of BKB's Supermod v2.0 installed over your copy of M:TW.

After you have gone through that, you can download and install the v2.1 patch. This is not required, but is recommended to fix gameplay bugs and similar issues. This process, however, is much more simple.

Firstly, lauch the installer and, as before, follow the instructions till it requests details of where to install the mod.

Type in "[hard drive letter]\Program Files\The Creative Assembly\Medieval - Total War - Gold Edition". Typically this will be "C:\Program Files\The Creative Assembly\Medieval - Total War - Gold Edition".

Select to continue, and the installation should proceed.

Once the process is complete, patch 2.1 should have installed normally over v2.0.