Is it possible to get mods onto Medieval II: Total War demo

No, you need the full game; I've tried it myself.

As for how to do it in the full game, I do not know.

FOR THIS TO WORK, YOU ALSO NEED THE 'KINGDOMS' EXPANSION PACK

The best place to get the mod is HERE. There are 3 separate files to download. Once they have downloaded, run each individually and save it to your Total War Medieval 2 folder

eg: C:\Program Files (x86)\Steam\SteamApps\Common\Medieval 2 Total War

Once there, you should have a third age folder. The easiest way to get it to run is to then rename the folder to one of the other DLC campaigns (I used teutonic as I don;t play it often), then cut/paste the actual teutonic folder elsewhere on your computer where you can retrieve it later should you want to.

Then you simply load up the game through steam, and load up the Teutonic campaign and Third Age will start.