Usually the digital downloads of a game from a authorized vendor like Steam or Gamersgate are the latest versions available. If you got your game via digital download and the game was later updated, you may have to obtain a patch file from the vendor as well to bring your game up to date.
The version numbers in Children of the Nile are "broken." They commited an error and never updated the "version" numbers in the game when they created new patches for it.
All I can tell you is that you must have Children of the Nile updated to the Enhanced Version (v. 1.3.01) before you install Alexandria.
If you bought your game between 2004-2008 in a box, you'll probably need the patch file on the immortalcities.com
website. Your game patch lanaguage version and the game language version must match. Example: If you bought in the U.S. get do not get the U.K. english patch get the U.S. patch.
If you bought of COTN through a digital download vendor like Gamergate between July, 2008 and now you probably have the enhanced version of the game and can install Alexandria. If you just bought Alexandria only and do not have Children of the Nile itself, go back to to your digital vendor, in your case Gamersgate, and purchase just digital download version of Children of the Nile and install it first. Then install Alexandria.