iWork was installed when you had 10.6 obviously, so it came on a disk and carried over when you upgraded to 10.7.
However the 10.6 iWork install disk likley won't work on 10.7, so you'll need to either contact Apple and see if they will give you a AppStore credit for iWork or something.
You can try using Pacifist from CharlesSoft, it can open packages on disks and grab programs to drag and drop install.
You paid for it so it's yours.