Using Terminal to resolve Verification block on OSX Mountain Lion please?

I am trying to install OSX Mountain Lion over Lion on a 2007 computer. A window opens telling me the software needs verification and I should go to the Apple Store and re-download software.

That is impossible as Apple Store no longer recognises this vintage software.

I did find the following advice:

Run this command in Terminal: xattr -cr "wherever your installer is, drag and drop into terminal and remove quotes.

OR xattr -c "wherever your installer is, drag and drop into terminal and remove quotes.

I cannot figure out what works. I need a clear sequence. I tried by dropping OSX installer into Terminal then typing xattr -cr. Terminal simply responded describing the the app as a directory. There were no quotes in the description.

I also tried downloading the Mountain Lion installer from other sources than Apple in the hope that a version difference bye-passed the issue. No success there either. My guess is that Apple only offer one version which all sites use.

I would be grateful for a Guru to describe the steps more precisely as I know nothing about Terminal instructions.