Open the sp_yourcityname_homecity.xml file in the My Documents
\My Games\Age of Empires 3\Savegame (dont forget to make a back up first) 
folder, search for [level].
You will see something like this [level]xx[/level], put in 99 for the xx, 
save the file, go into the game and play a skirmish. Now you are level 99 !