first, you go to the map "data".
now go to the map "text".
use a standard text editor (like notepad/word etc.) to open "descr_shortcuts"
go down until u see "misc." (at the start of the line)
there should be a line with "show_console" here, and it says "hidden" behind it.
remove "hidden" and save the file
start Rome: Total War, go to controls and press reset to default
there should now be a line "show debug console" in the list of misc. controls
set whatever key u like to it, press the key during the game, and there you go, a nice console"