To clear old versions of ISIM (or other webstart applications) from the web
start cache on the Mac:
  1) Find "Java Web Start.app" and launch it

  Try step 2a, below, first.  If that does not work, go to step 2b.

  2a) In the "Java Web Start Application Manager" window, select "Ion Simulator Interface" (or whichever application you are trying to update).
  3a) Select the menu Application->"Delete Application"

In case you were unable to remove the application from the web start cache using the above steps, try the following:
  2b) Select menu "Java Web Start"->"Preferences..."
  3b) In the Preferences dialog, "Applications Folder Options" section, click "Clear Folder".  This will remove the local versions of all installed web start applications.