The TW site (oddly enough

) has the necessary patches to patch your game. If you have M2TW without Kingdoms, then you need to patch to 1.3. Go to
http://www.totalwar.com/ . In the left hand column are drop down menus. Go to Support, then Medieval II. It says that you need 1.2 and then 1.3 to patch the game.
For Kingdoms and Gold users, this is slightly different. Again go to the
http://www.totalwar.com/ site. Click Support in the left menu, then Kingdoms. If you have the gold edition of M2TW and Kingdoms, then you need to download one of the "Gold" mirror patches. If you do not have the "Gold" edition, then you should download one of the patch mirrors that lacks this word. This will update your game to the correct patch number.