Hi,
Not quite like that...
You, as the developer, get a Key from Google that is linked to your
site. You then embed code in your web page that creates an instance of
the GE Plugin - in which your users will be able to view any
additional content you add.
However if your users do not have the Plugin installed they will be
prompted to install it. You don't need to write code to handle that it
is done for you when the Plugin tries to initialise - they will get a
Load Failed message and then an install button over the Plugin div
that installs the Plugin (if it is supported by their browser).
To play a KML/KMZ within a browser the user has to have the Plugin
installed locally - it doesn't matter whether they have the full
Google Earth application or not - the two are unrelated. (They can of
course open the KML/KMZ in the full app if you provide a link for then
to do that - but that has nothing to do with the API key etc - it's
just a file download).
Content and the Plugin are different things. Content (KML/KMZ) can be
loaded into either the full application or the Plugin (if you write
the code for it to do that). The Plugin is just a stripped down
version of the full app that can be embedded in a browser - stripped
down in that you, the developer, have to write the code around it to
display the content you want within it.
Hope that helps and doesn't muddy the waters.
Regards
Nymor