I have the same problem with retrofit. I can't get it to install. The reg hack was pretty easy, so that would be cool to have for other steam users.
Here's the contents of the original rag hack that worked. Just save it to a ./reg file and run it. If someone can let me know what values to change that would be great.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Sega\Medieval II Total War\Mods\Unofficial\Stainless Steel]
"Fullname"="Stainless Steel"
"DisplayName"="Stainless Steel"
"Version"="4.1"
"Author"="King Kong"
"DistributionSource"="download"
"Path"="Stainless_Steel_Mod"
"ConfigFile"="Stainless_Steel_Mod.cfg"
"Language"="English"