For those who have steam and have purchased rome total war gold and the alexander expansion, and wish to get this mod working without the "cannot find steam" error, Ive got instructions as to how to make it work (if this has been posted before, feel free to delete this post as it is obsolete).
After installing the mod as usual (into rome total war gold folder),
open steam's games menu, right click "Rome : Total War Gold", and click on "Create desktop shortcut". Once its done, right click the shortcut youve just created and select properties, there will be a line that says something like :
Target :
"C:\Program Files\Steam\Steam.exe" -applaunch 4760
(at least, thats the target path for me).
Copy that line, then right click on your "Roma surrectum" shortcut on your desktop, and select properties, then overwrite the target path that has been placed there by the install.
Now what this will do is that whenever you will click on the "Roma surrectum" shortcut, the mod will launch through steam while using the RTW original exe (it doesnt use the BI one).