How do I skip verification of a .dmg file? I have downloaded a dmg file but when I click on it it keeps verifying. 

I've also tried to go to /System/Library/CoreServices/Installer.app but I can't click on the .dmg because it is greyed out.

How can I open it?
The command line tools allow you to mount the DMG and skip verification.
Open Terminal:
hdiutil attach -noverify /Path/To/Your/DMGFile.dmg

If you have the DMG file on your Desktop, and the DMG was named &quot;Broken.DMG&quot; you could type in something like this:
hdiutil attach -noverify ~/Desktop/Broken.dmg

Rather than typing the name of the file, just drop the file icon in the terminal window once the main command is ready. If your image is bad, the warning is correct and you should get a new dmg rather than bypass the error checking step.

A more global change would use defaults, but this check is very handy in general since it flags bad images:
defaults write com.apple.frameworks.diskimages skip-verify TRUE

This will stop the verification of DMG files permanently. After you've managed to skip the verification and open that file, it would be a good idea to re-enable the verification by opening Terminal again and typing in:
defaults write com.apple.frameworks.diskimages skip-verify FALSE