You don't. AppleWorks hasn't been supported for years, and does not work in any systems from Mac OS X 10.7 and on. You will either need to downgrade to Snow Leopard or older (which will not be possible if you bought a new machine with Mac OS X 10.8 preinstalled) or find other apps to open your AppleWorks files. Apple's iWork suite (Pages, Numbers and Keynote) can open some AppleWorks files. Those apps are available in the App Store. Unfortunately, nothing but AppleWorks can open an AppleWorks database, as far as I know.
Additionally, if any of your files were created with the painting app of AppleWorks, I have not found anything that can open that either. Since I still use that mostly for backgrounds of artwork I create, I maintain a bootable partition of Snow Leopard so i can use AW. You'd need to export your files in a .pdf or .jpg format to open them in Mountain Lion.
My external Harddisc has gone belly up, My Applworks app was on it. All my Appleworks docs now have blank icons. Is there any way to recover these many, many docs?
If I can buy AppleWorks6 app will installing it recover the docs?
I have many docs spread over two external harddiscs and some on my internal HD.
You cannot install AW on a Mountain Lion (or Lion for that matter) Mac.
You would need to have a bootable partition running Snow Leopard on a Mac that originally came with Snow Leopard to do anything with AW. Also not at all sure that you can still buy AW (although I did see one at Amazon for an outrageous price).
How do I open Appleworks6 in it?
1. Read Roger Wilmut's excellent series of articles - Abandoning Appleworks:
2. Consider running Appleworks 6 in Snow Leopard Server installed in Parallels 8 in Lion or Mt. Lion:
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Apple is now selling Snow Leopard Server for $19.99 + sales tax & shipping costs at 1.800.MYAPPLE (1.800.692.7753) - Apple Part Number: MC588Z/A (telephone orders only).