The Squish IDE is configured to use one specific Squish package. The configuration is done by Squish's setup program---this program is automatically run by the Squish IDE the first time the IDE is run.
It is possible to make the Squish IDE use a different Squish package (e.g., a newer one, or one for a different GUI toolkit). First, click the Window→Preferences menu option to invoke the Squish Preferences dialog.
Once the Squish Preferences dialog is visible click on the Squish item in the list on the left. At this point the dialog should show the Squish panel and in this panel the list of Squish packages that are available.
Simply click the package you want the Squish IDE to use from now on and then click the dialog's OK button to confirm your choice.