When I try to reinstall "IDM integration" extension, Chrome says Google has flagged "IDM Integration" as malicious and installation has been prevented

First of all this is a false identification from Google.

Internet Download Manager and its components do not have any worms, viruses, trojans, spyware, malware, adware, or other malicious parts inside of it.

Steps to resolve the problem:

1. Please update IDM to the latest version by using "IDM Help→Check for updates..." menu item.

2. Please restart Chrome and enable "IDM integration" extension in Chrome manually.