Follow these steps to get Expression Master running in After Effects.
Get the latest .zxp file from the home page. Make sure you have a ZXP installer like ZXPInstaller or AEScripts ZXP Installer.
Open your ZXP Installer and drag the ExpressionMaster.zxp file into the window. Follow the prompts to complete the installation.
In After Effects, go to Edit > Preferences > Scripting & Expressions and ensure "Allow Scripts to Write Files and Access Network" is checked.
Restart After Effects. You can now find Expression Master under Window > Extensions > Expression Master.