Unable to install SOA Composite Editor extension in Jdeveloper 11.1.1.9

I needed to review the code for an Oracle SOA Composite application on a computer where I had previously uninstalled Jdeveloper 11.1.1.9 so I reinstalled the IDE.

While trying to update jdeveloper with the SOA Composite extension, I noticed the following:

1. The extension was not listed as available for install after using the Help > Check for Updates menu and clicking Next twice (once to choose the Update centers to check, and the second to actually select the desired extension). Where did it go? This is where I had seen it before. Had it been discontinued? How else was I supposed to develop Oracle SOA Composite applications now? I proceeded to download the extension to the computer for an offline installation. But...
2. While trying to install the extension offline, it failed with the message that some components were missing from jdeveloper. The dialog box with the error message listed two such components. This happened even after reinstalling (on mac os x with apple java 1.6 to run the installer).

How to fix this?

In the .jdeveloper folder in your home directory, delete the system folder for jdeveloper 11g. For me it was named system11.1.1.9.40.66.73. 
The folder will be recreated on restarting jdeveloper.

And installing the extension either way should proceed normally.