MacOS – I have El Capitan and can not install a DMG file

First, mount the dmg image : sudo hdiutil attach <image>.dmg

The image will be mounted to /Volumes/<image>. Mine contained a package which i installed with: sudo installer -package /Volumes/<image>/<image>.pkg -target /

Finally unmount the image: sudo hdiutil detach /Volumes/<image>.

The error message means that the downloaded file is corrupt and can't be opened.

Re-download it and try opening it again. If it fails again, make sure that the download isn't being interrupted.

If you can obtain a checksum for the file, you can check it against your download by running openssl sha1 ~/path/to/download which will show the checksum for your download. Comparing the results should show if the file you downloaded matches what you should expect to download.