After spending 3 days searching and trying different ways to get the Third Age mod working in my Medieval II game, I finally got it working! Thought I'd post here for others - maybe it'll help...
First, you have to download the files - Third Age - Total War 3.0 here: http://www.moddb.com/mods/third-age-total-war/downloads
The mod has a part 1, part 2 and a patch. I haven't tried the patch yet, so not going to include any info on that just yet...
Once you've downloaded parts 1 & 2, you have to install them - double click on the file and run the installer. The default installation is into a folder called SEGA. You have to change this to reflect your mod folder in the Medieval II game - usually it's: C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods
Installing part 1 creates a folder called Third_Age_3 in your mods folder.
The part 2of 2 file is missing the .exe extension, so if you try double clicking it, nothing happens. Rename it and add the .exe extension. Then double click it and install in the newly created Third_Age_3 folder - remember you have to do this by changing the destination folder during the installation.
Once you've done this, you have your mods in the Third_Age_3 folder. So, how do you get them to work in Steam?
Open Steam and right click on the Medieval II Total War game from your Steam Game Library and choose properties.
Click on "Set Launch Options"
Add this to what's already there, after the quotation marks: --features.mod=mods/Third_Age_3
Click OK and close.
Click Play to play your Medieval II game and choose the option "Play Medieval II: Total War"
Third Age should launch.

I spent way too much time trying to figure this out, and wish I had steps such as this posted somewhere to help me - hope this will help someone else and save them a lot of time and headaches!!