As people had issues installing the script, I’ve made a quick screencast that shows how to download, install and use the script:
As shown in the screencast, it’s easiest to make use of the direct “download” link that’s available in the “Installation” section of the script’s Readme.
After download has completed, just double click the .zip package to unzip it. Then move the .scptd file to the DEVONthink script folder that’s located at ~/Library/Application Scripts/com.devon-technologies.think3/Menu.