Right Click
Select "Open With"
Open with Note pad.
You can now edit the file.
Also don't edit the original file. Copy it, and place the copy you intend to mess with in the corresponding folder in your Custom's Assets folder. That's in your My Documents Directory, rather then program files. The game will then use this file (and any other tweaks you make to the game in corresponding files) when loading, but you save the original in case you make a mistake, or want to play multiplayer. Just make sure you place the file in the corresponding directory in your custom assets folder. So if the file you're altering is C:\Program Files\Fireaxis\Beyond The Sword\Assets\XML\foldername\file Then copy the file and place it so that the file you edit goes in C:\My Documents\Games\Fireaxis\Beyond the Sword\Custom Assets\XML\foldername\file
Welcome to modding CIV. You'll find you can easily change alot of things this way, and if you screw up, it's so easy to fix, you just delete the file in your custom assets folder, and the game will use your original in the Program Files directory.