Download the Mega-age mod here:
http://67.66.187.69/dominions3/mega-age.zip
You will need to extract these files/folders to your dominions3/mods folder. You will then be able to enable/disable the mod via the dom3 game menu > Preferences. You do not need to enable this mod to upload pretenders.
This info has also been posted to the first post in this thread.