Trouble Installing-nothing happens.

Hi.
opened LW4.0.7PRO today, and got a notice re a newer version.
Great!
downloaded AOK via Safari
dmg opens, but when i click on the installer, nothing happens.
the icon zooms up, as expected, so it looks like something is about to happen, then nothing does.

do i need to un-install my previous version?
i moved the App folder to an external drive, but that didnt help.

re-booted,
permissions have been repaired, and are sqeaky clean.

any help appreciated,
thanks,
nm