You need to unpack those files with 7zip (free compression tool).
Just open the first one in 7zip and unpack. It should automaticaly include both parts 001 & 002.
Then Install Stainless_Steel_5.0 & afterwards Stainless_Steel_5.1b_patch into the medieval folder: C:\Program Files (x86)\SEGA\Medieval II Total War or for steam users C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War.
Also, when playing on steam, you need to rename Stainless steel folder in mods folder ---> (C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods) after one of the expansions to run it (for example americas). Then selecting americas expansion in launcher SS5.1 runs instead. Good luck!