May 25, 2010 at 4:36 PM
Edited May 25, 2010 at 4:37 PM
There are two scenarios that you cannot see "Unblock" button under General tab->Security row at the bottom (Win XP only):
1. That chm file has been unblocked (Once it is unblocked, "Unblock" button will be gone.). Obviously you are not in this case since you still cannot open it.
2. You probably missed the permission to view the entire contents of the General tab. Log in as local admin and see if you can see "Unblock" button there.
It is not documented, but I can reproduce the problem you had on win xp.