Ok. I'll do my best. About a week ago, I used my CS4 Master Collection installer to install PS and DW. I had no issues. I don't use IL much, so I hadn't installed it. However, a font designer I'm working with asked that I send him an IL file of what I was looking for, so I went back into the installer to install IL. I double-clicked Setup. It asked me for my admin user name and password (this is the standard OS dialog, not Adobe's). I entered my user name and password. (I'm the only user, so I am the Admin). Setup stayed open just another few seconds and quit without any further error, or dialog.
I tried this several times, all with the same result.
I hope this is enough to get you started. Sorry I wasn't clearer before
It sounds to me like it is choking when it tries to display the license agreement, maybe because it can't display one of the fonts used in it.
Are you using a font manager? If so, turn off all fonts in it and try again. Or better yet, active just the default Apple fonts.