Under Ubuntu 12.0.4 I have loaded a basic sketch to test the installation (UNO, it works) but I can't edit the sketch within the IDE, the way I can under Windows. I can move my cursor over the code but I can't change it. What am I missing?
Open the IDE. Go to File, Preferences. Make sure "use external editor" is unchecked.
Thanks - The "external edit" box was not checked; I checked it and it does not work
It might be "unclutter" (if installed) getting in the way. I have issues with it causing problems with Thunderbird in 13.10 in that focus doesn't always work right.
Try, from a terminal:
$ killall unclutter
and see if that improves things.
Making the editor window read-only when a read-only file is open is a common behaviour for editors. I don't know whether the IDE does that on Ubuntu but that would be my first guess.
If you want to change one of the examples, use file / save as to save a copy in your own sketch folder and then edit that.