Taken from TWC:

Use the run command, or command prompt, and type "regedit"


[HKEY_USERS\[Your String]\Software\The Creative Assembly\Empire]
"ep_unlock"

to 5

This will allow you to play any RtI episode.

Warning: Registry Editing is not for the computer illiterate. You can cause a lot of harm, if you stick to this one key, you will be fine.