I click to install addon, DL completes, attemptes to Un-Zip then it says "Instalation Failed" why is that?

There could be a number of things that might give that error.  Anywhere from a zip program that is not working properly, to attempting to write to a section of your hard drive that is having issues, such as bad sectors.  First I would suggest to uninstall your current zip program.  Run a disk defrag on your hard drive and then download and install a good zip program.   Those are pretty easy to check and confirm it works or doesn't.

If you are using the Curse Client, file a ticket. Let them know what's happening. Worst case: Uninstall your client, and download and re-install it. In the meantime, try a manual install of the addon you want.