Make sure to install the game with this:
and launch it with the launcher from that installer. If that doesn't work then try using residual (http://residual.sourceforge.net/
And it's not really wise to openly state that you downloaded the game even if it is hard to find retail.
This thread also belongs in: http://www.lucasforums.com/forumdisplay.php?f=236
which is for technical problems with the game.