Locate the sp_NameOfCity_homcity.xml file in your \...\Age of Empires 3\savegame\ 
directory. Edit or open the file using Notepad or a similar text editor. In the 
file, search for the text similar to the string and you should find a list of all 
the cards available for the game. 
Place the technology card you want in between the and section and you should be 
able to play any of the cards you want. Save the XML file and boot the game. 
Note that normal game restrictions of having only eleven cards is still in effect.