You can access these options from the menu Tools » Options.

Confirm before deleting	When checked, when you attempt to delete an object, a popup dialog will ask you to confirm.
Automatically check for updates	When checked, periodically look for KML Editor software updates.
Popup Messages...	Opens a dialog where you can turn popup messages on or off. Only popup messages where the Do not show again check box will appear in this list.
Show balloon text	When checked, the first 2 lines of the description text will appear in the snippet if the snippet has no text. Leave unchecked to ensure your kml file only shows snippet text.
Automatically edit	When Google Earth is opened and while adding a new Feature, if the selected Tab is Preview and the 3-D Earth Viewer is docked in KML Editor, opens the Feature Properties Dialog allowing newly added Feature to be moved around the Earth.
Preview your changes	When checked and Google Earth is open, the Google Earth 3-D Earth Viewer is automatically docked in KML Editor Preview Tab.
Single click preview	When checked and Google Earth is open, reloads the kml file into Google Earth each time a Feature is clicked in the My Place Tree. Press and hold the CTRL key when clicking other Fetaures in KML Editor My Place Tree to prevent Google Earth from flying-to-view.
When KML Editor Starts
Launch monitor	When checked, launches a Google Earth monitoring helper into the system tray to help you recover in case KML Editor doesn't return Google Earth to its normal state. You can also prevent the monitor popup balloon text to show each time it starts monitoring.
Launch Google Earth	When checked, opens Google Earth automatically if it isn't already open.
Open last edited file	When checked, opens the last edited file.
Close Google Earth	When checked, closes Google Earth when KML Editor closes.