Google provides a few extensions to the standard KML language. One such extension is that exact feature. Adding the element <gx:balloonVisibility> with a value of 1 to your Placemark will make the description balloon appear immediately when the KML is loaded.
Note the gx: namespace prefix to the element, which is is required as well as the xmlns:gx declaration at the top of the KML file as shown in this example.
Example:
<kml xmlns="http://www.opengis.net/kml/2.2"
xmlns:gx="http://www.google.com/kml/ext/2.2">
<Placemark>
<name>Eiffel Tower</name>
<description>
Located in Paris, France.
This description balloon opens
when the Placemark is loaded.
</description>
<gx:balloonVisibility>1</gx:balloonVisibility>
<Point>
<coordinates>2.294785,48.858093,0</coordinates>
</Point>
</Placemark>
</kml>
Reference:
https://developers.google.com/kml/documentation/kmlreference#gxballoonvisibility
BUG:
This feature worked in older versions of Google Earth, but is broken in Google Earth v7.1.2.2041 (at least confirmed on Windows) if GE launches with such KML it won't auto display the balloon but if GE is already started then opening the KML will auto-popup the balloon description. A bug report has been filed with Google.