Look at this CodeProject article:
Embedding Google Earth in a C# Application[
^].
As no downloadable source code is provided, it's not so easy to understand where to get the library and what should be referenced. According to one of the answers in the "Comments and Discissions" section, one could understand: you can install Google Earth desktop application and reference "googleearth.exe"; as far as I can understand, as a COM component. I did not try. Please give it a try. If by some reason it does not work, ask the author of this article.
—SA