Fred and others,
Latest update on my long journey to Linux.
After re-verifying the MD5Sum and burning the CD with the program Oliver recommended, using the slowest speed, I tried to install again.
I was amazed that the new Live CD did not behave the same way as the previous one. On startup, it did some stuff that the previous one didn't do, like there were some command-style, white text on black background screens that popped up, saying "Opening..." this or that process. There was also a login screen that wasn't there previously, which was baffling because this was the Live CD and I didn't have any username yet. But I just ignored it and eventually the desktop appeared. It did seem to take longer to appear than with the previous CD.
All these differences with the previous CD led me to believe that Fred was correct, and the old CD was a bad burn. It did seem very weird to me that the old CD would run at all if it was incorrectly burned.
So I tried to install again and, in the edit partition window, ran into the same problem -- the Mount Point drop down menu was dimmed.
I thought that maybe the problem was with the original partitioning, which was done from the faulty CD. So I decided to go back to Gparted and deleted all the old partitions I had made, and created new ones. I also didn't give them labels, thinking that maybe the labels were the problem.
Then I tried to install again. When I got to the manual partitioning part, i was horrified to see that the Mount Point menu was still dimmed!
But then I pulled down the drop down menu for "use as" and set it as ext3.
Suddenly the Mount Point menu become undimmed and I was able to select "/"
But I am sure that I tried that with the earlier CD, and the mount point menu did not become undimmed! But maybe I'm not remembering correctly.
Anyway, I was finally able to install. Immense thanks to everyone, especially Fred.
I still have to mount those data partitions, and have one or two new problems, so I'm sorry to say you haven't heard the last of the whining newbie Gumby!