Once installed, IDM works automatically:
Your extension should now appear in the toolbar. Clicking on it should display a popup with the message "Hello, World!". idmgcext.crx
If the extension didn't install automatically with the IDM software, you can add it manually using these steps: idmgcext.crx
You must have the Extensions page open and Developer Mode toggled on for the drag-and-drop to work. idmgcext.crx