You need to download and install some game packs that add some different
units, resources and civilizations to the game.
You can get the install packs at the following links:
GOTM 21 Full setup:
http://gotm.civfanatics.net/games/saves/gotm21_v129_full_setup.exe
GOTM22 Add-On Setup:
http://gotm.civfanatics.net/games/saves/gotm22_v129_add_on_setup.exe
GOTM23 Add-on Setup:
http://gotm.civfanatics.net/games/saves/gotm23_v129_add_on_setup.exe
GOTM24 Add-On Setup:
http://gotm.civfanatics.net/games/saves/gotm24_v129_add_on_setup.exe
GOTM 25 Add-On Setup:
http://gotm.civfanatics.net/games/saves/gotm25_v129_add_on_setup.exe
All you need to do is download these, and then install them in the order
that you download them. The key point is to make sure that when the
installer asks where you want to install to, that you make sure that it
installs to the directory where you installed Civ to in the first place.
You will also find that when you try to play 'normal' games, the resources
pictures will be all messed-up. This is because the GOTM installs a
different resources picture. Unfortunately, this means that when you want
to play a normal game, you need to swap yuor resources.pcx file.
The GOTM install package actually provides a readme file on how to do this
(its fairly simple), but if you have any problems with either the
installation of these files, changing the resources.pcx files or any error
messages playing the game of the month, then please don't hesitate to e-
mail or send a PM to myself or one of the GOTM staff.
You will also need a new pediaicons.txt file, which you can download from
this post