I'm not sure what you mean, either. Do you literally mean "prompt" a
user, or do you want to give the user an option to load from a pre-
determined list of kml's that are available?
For the first scenario, see Roman's Demo in the Demo section on
loading the KML dom. I think it's on the 2nd or 3rd page of demos.
There, he let's you fill in the path of any kml from anywhere on the
web. If you fill in a non-existent one or a bad file, it will, of
course, fail to load it.
For the second scenario, you are welcome to look at my page source at
http://sovereignstates.net/StateLegislators/VSLegislators.htm
My source might not be as pretty as some, but the logic is there and
seems to flow well. Plus, I am still working on it.... Showing and
hiding layers works well, though.