I should also mention that I am using Mac OS X.9.2

Assuming that you have downloaded the keyconfig XPI installation file already (e.g., with Firefox from here as it's not hosted on the official add-ons site), open the Add-ons Manager from the ≡ application button or the Tools menu. Then, either just drag-and-drop the XPI file from your desktop or a folder into the Add-ons window or use "Install Add-on from File" from the "gear" menu next to its search box. This should initiate the installation process.

Ok. Thanks.

I did this, and I a modified "redo" as command-Y, and then changed "get new mail" in order to avoid a conflict.

However, it doesn't seem to work when I type an email.

modified "redo" as command-Y - that should be the default keyboard shortcut?

I'm not using keyconfig myself, thus can't help much further. If you follow the link I've posted above, that's the forum thread where the author provides support.

In the Mac, the command for "redo" is shift-command-Z. I have no idea why.

In any event, "redo" is changed in the Edit menu, and it works by clicking on it. But not as a key command.

The only explanation that I could find was in this bug report, stating that Cmd+Y doesn't work as shortcut on OSX in the Composition window for some reason. You may be running into that issue if you try to force it using keyconfig, thus you may have successfully redefined it but it doesn't work anyway due to that conflict (unless "Get New Mail" is the one causing the conflict and you are trying to resolve that in the same step).

I tried to redefine "get new mail," but yes, it looks like I'm being stopped by this bug.

Frustrating. But thanks.

You are welcome. Maybe someone else stops by with a suggestion.