I found thisone and it helped me out :-)
>>>>>>>>>>>>>
sandersk
This is what FINALLY worked for me after reinstalling and uninstalling and doing everything else mention above, and with a call to Apple's Support.
Move the applications into the trash (of course) and then make sure you remove the folder called "iWork '09" in your Hard Drive Library (not your User Library) under Application Support. You can throw away your preferences too if you want, but I didn't. Then do a clean install. And THEN open the Mac App store and run the auto update (or just select Software Update from the apple menu).
At first I thought it was too simple of a fix, but it indeed fixed everything. All iWork apps opened, opened files (they were saying they couldn't be opened) and Keynote finally worked (it was crashing and erroring out). I can now use all iCloud features, and everything, no problem.
I am pretty tech savvy and felt silly for not even thinking of this and how easy it was to fix. I even used an app uninstaller to make sure it removed all of iWork's little bits, but it didn't do that last folder. It has been causing problems for a week and am so glad I contacted customer support. You can contact them if you just got Mountain Lion. Just sign up to have them call you or give them a call if this doesn't work for you.
Would love to hear if this worked for other people.