Assets > UI > FrontEnd > MainMenu.xml
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<Button ID="QuickStart" Anchor="L,C" Style="MainMenuButton" String="QUICK START" />
<Button ID="Skirmish" Anchor="L,C" Style="MainMenuButton" String="CUSTOM GAME" />
<Button ID="Campaign" Anchor="L,C" Style="MainMenuButton" String="CAMPAIGN" Disabled="1" />
<Button ID="Multiplayer" Anchor="L,C" Style="MainMenuButton" String="MULTIPLAYER" />
<Button ID="LoadGame" Anchor="L,C" Style="MainMenuButton" String="LOAD GAME" Hidden="1" />
<Button ID="Options" Anchor="L,C" Style="MainMenuButton" String="OPTIONS" />
<Button ID="MapEditor" Anchor="L,C" Style="MainMenuButton" String="MAP EDITOR" Hidden="1" />
<Button ID="Exit" Anchor="L,C" Style="MainMenuButton" String="EXIT" />
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You can enable the Map Editor by changing Hidden from "1" to "0".
I can't actually test it out, as the game crashes for me since the hotfix as soon as I get to a loading screen. But I did get as far as choosing the map parameters and beginning to load the map, so I think this works.
Changing Disabled to 0 on the Campaign takes you to a menu where you can choose a campaign, but clicking on a campaign button does nothing.