Open the File Explorer window where you found the IDMGCExt.crx file.
Navigate to the IDM installation folder. By default, it is:C:\Program Files (x86)\Internet Download Manager download idmgcext.crx file for opera
Click and hold the IDMGCExt.crx file, drag it over to the Opera window, and drop it. Open the File Explorer window where you found the IDMGCExt