In RH7 it was enabled but did not work if the index was open at the time. Close the index and run it, then it worked. Not sure if and how you could restrict it to one index.
In RH8 it does appear to be completely broken.
http://www.adobe.com/go/wish to report it.
See www.grainge.org for RoboHelp and Authoring tips
In my experience in RH7, it was only enabled if the Index tab was open and I clicked in it first. Then the menu item was enabled. If the cursor focus was anywhere esle, the item did not appear on the menu...
I've sent this off to Adobe as a possible bug.
Thanks for your help.
The Remove Unused Index Keywords function does not work in RH8. When I have the index open, it is highlighted and I can select it but nothing happens. I can scroll through and find unused index keywords or run a report to find them. I hope they fix this!
I have now.