Overlord is an extension. This means it's installation is a little different than a script like RubberHose.
To reduce the complexity of installing extensions, the smart folks at aescripts + aeplugins built a really awesome Extension Installer that will make your life a lot easier to get up and running with Overlord. So download and install it.
Drag Overlord.zxp into ZXP Installer
Close and reopen After Effects
Open the Window menu, find the Extension group and you should see Overlord
Be sure to open the Overlord panel in both After Effects and Illustrator
If you follow all the instructions of the standard installation and Overlord still isn't showing up in the Window>Extensions menu you're gonna need to do a manual install. You didn't do anything wrong, it just happens sometimes.
Change the extension of the Overlord.zxp file to .zip. It might give you a dialog warning about changing the file type, but ignore it.
Unzip this file to get a folder called Overlord
Navigate to the Adobe extensions folder:
[Mac]: /Users/**username**/Library/Application Support/Adobe/CEP/extensions/
If you don’t see this folder on a windows machine, try these:
Windows x64: C:/Program Files (x86)/Common Files/Adobe/CEP/extensions/
Windows x32: C:/Program Files/Common Files/Adobe/CEP/extensions/
Copy this Overlord folder to the /extensions/ folder
Restart Ai/Ae and see if you see Overlord in the Window>Extensions folder
Immediately after purchasing from Gumroad, you'll receive a license key to unlock Overlord. Open the panel and paste this key and you'll be ready to work.
It's a good idea to create a Gumroad account so you have an easy way to to download the installer again or retrieve your key from the Gumroad Library.