Well, I don`t have steam version of M2TW, but it should work if you copy\paste files into your America's(Thirdage) folder, and overwrite everything when asked.
And I forgot to mention, when you install TATW 3.1 patch and point it to install to your DESKTOP, it makes a data folder on your desktop, and inside is this models_missile folder which goes to your steam\...\ Medieval 2 TW\ data folder :
And when you install TATW 3.2 patch to your DESKTOP, it creates a mods folder on your desktop, open this mods folder and copy paste content of Third_Age_3 folder to your steam\...\ Medieval 2 TW\ mods\ America`s ( Third Age) folder ( overwrite everything when asked ):